determinant means a withdrawal of nourishment, however slight, from its
neighbours; this can, of course, be equalized again if the claims on
the common nutritive stream from another quarter are at the same time
diminished; but it is possible that the claims from another quarter
may also be increased, and the withdrawal will then be more marked,
and the determinants being thus injured from two directions at once
will sink downwards with greater rapidity. But it is also conceivable
that the majority of determinants of a part may vary upwards, and,
by their combined increased power of assimilation, direct towards
themselves such a greatly increased stream of nourishment that the
whole organ--for instance, a particular feather in a bird--varies in an
upward direction, and becomes larger and larger, as we see in the case
of many decorative feathers; or that certain determinants vary only as
far as some of their biophors are concerned, and similarly for their
determinates, as when a group of scales on a butterfly's wing that had
previously been black turn out a brilliant blue. It can probably also
happen that such variations within the determinants are transmitted to
neighbouring determinants because the nutritive conditions which caused
the first to vary have extended to those about them. The increase of
brightly coloured spots in birds and butterflies gives us ground for
concluding that there are processes of this kind within the germ-plasm.
I will refrain from following this idea into greater detail, and
translating the observable relations and variations of the fully-formed
parts of the body into the language of the germ-plasm; but so much may
be taken as certain, that multitudinous inter-relations and influences
exist between the elements of the germ-plasm, and that one variation
brings another in its train, so that--usually at a very slow rate,
that is, in the course of generations and of species-forming, definite
variations occur from purely intra-germinal reasons--variations
which as far as they remain outside the limits of good or bad may of
themselves change the character of a species, but which when they are
seized upon by personal selection may, by sifting and combination of
the ids, be led on to still higher development.
If we consider further that the variation of a part must depend
not only on the quality of the external stimulus but also upon the
constitution, the reacting power of the part, we shall understand that
similar nutritive variations may cause two different determinants to
vary in different ways, and when we reflect that every nutritive change
must extend from the point from which it started with diminishing
strength in a particular direction, we have a further factor in the
variation of determinants and one which influences even similar
determinants differently.
Finally, if we remember that determinants of different constitution
will also extract different ingredients from the nutritive stream and
thus set up in it different kinds of chemical change, thus causing
an altered supply of nutritive substances to flow to the neighbour
determinants, we get some insight into a very complex and delicate
but perfectly definite set of processes, into a mechanism which we
can certainly only guess at, but whose results lie plainly before
us in the spontaneous variations of the organism. We understand in
principle the possibility of saltatory variation, as a more or less
widespread, more or less marked disturbance of the species-type in
this or that group of characters, and we may acknowledge that those
'kaleidoscopic variations' which Eimer supposed to be the sole basis
of the transformation of species, and which have been brought to the
foreground again quite recently by De Vries[20], are probably factors
in transmutation operative within a limited sphere.
[20] See end of chap. xxxiii.
But we must think of all these struggles and mutual influencings as
taking place on the smallest possible scale, so that it is only by
long summation that they can produce any visible effect, and we must
never forget the essential significance of the plurality of ids, for
these 'spontaneous' variations may take place in a different and quite
independent manner in each individual id. If this were not so no
intervention of personal selection would be possible, natural selection
would not exist, and the adaptation of the organism from the single
cell up to the whole would remain wholly unexplained. The whole crop of
spontaneous germ-variations, whenever it ceases to be 'indifferent,'
and becomes either 'good' or 'bad,' comes under the shears of personal
selection and under its almost sovereign sway.
On the other hand, the sudden first appearance of a saltatory variation
takes place quite independently of personal selection, depending on
similar variations in a number of ids, which remain latent until they
have by the process of reducing division which precedes amphimixis,
chanced to attain a majority. In sudden bud-variations we may perhaps
suppose that reducing division occurring in some still unverified
abnormal manner is the reason why the germinal variation suddenly makes
itself visible--a supposition previously suggested as the explanation
of the reversion of these sports.
The rarity of bud-variation is thus explained, while the greater
frequency of saltatory variations in plants propagated by seed may
be accounted for by the regular occurrence of reducing division in
sexual reproduction. But that the same or similar variations may
occur in several, it may be in many, ids at the same time must depend
upon similar general influences which affect the plant as a whole, as
happens through cultivation, manuring, and so on. I shall return to
this when discussing the influence of the environment.
In some quarters this whole conception of germinal selection has been
characterized as the merest figment of imagination, condemned on this
ground alone, that it is based on the differences in nutrition between
such extremely minute quantities of substance as the chromosomes of
nuclear substance within the germ-cell. The quantity of substance
is certainly minute, but it needs nutriment none the less, and can
we believe that the stream of nourishment for all the invisibly
minute vital elements is exactly alike? It may be admitted that the
nourishment outside the ids is usually abundant, although undoubtedly
fluctuations occur in it also, but it certainly does not follow from
this that every vital unit within the id is similarly disposed in
relation to the nutritive supply, or has food in equal quantities at
its command, or even that each has as much as it can ever need. To make
an assertion like this seems to me much the same as if an inhabitant
of the moon, looking at this earth through an excellent telescope and
clearly descrying the city of Berlin with its thronging crowds and its
railways bringing in the necessaries of life from every side, should
conclude from this abundant provision that the greatest superfluity
prevailed within the town, and that every one of its inhabitants had as
much to live upon as he could possibly require.
We certainly ought not to conclude from the fact that we cannot see
into the structure and requirements and methods of nutrition of a
very minute mass of substance that its nutrition cannot be unequal,
and that it cannot, by its inequalities, give rise to very material
differences, especially when we are dealing with a substance to which
we must attribute an extraordinarily complex organization built up of
enormous numbers of extremely minute particles. That this complexity
is undeniable is now admitted by many who formerly thought it possible
to believe in the simple structure of the germ-substance. How complex
not only the germ-substance but every cell of a higher organism is
in its structure, and how far below the limits of visibility its
differentiations and arrangements reach, is pressed upon our attention
by the most recent histological researches, such as those we owe to
Heidenhain, Boveri, and many others. The whole scientific world was
amazed when it came to know the mysterious nuclear spindle in the
seventies, and since then this has been quite thrown into the shade by
the discovery of the centrosphere, the centrosome, and more recently
even the centriole, and now we believe that these marvellous centres
of force may, or must, possess their own dividing apparatus! In the
face of discoveries like these no one is likely to be able to persist
in recognizing as existing only what is disclosed or even hinted at by
the most powerful lenses; no one can any longer doubt that far below
the limit of visibility organization is still at the basis of life,
and that it is dominated by orderly forces. To me, at least, it seems
more cogent to argue from the phenomena of heredity and variation to
an enormous mass of minute vital units crowded together in the narrow
space of the id, than to argue from the calculated size of atoms and
molecules to the number which we are justified in assuming to be
present in an id. In my book on the germ-plasm I made a calculation of
this kind, and I arrived at figures which seemed rather too small for
the requirements of the germ-plasm theory. This has been regarded as
a proof that I disregard the facts for the sake of my theory, but it
should rather be asked whether the size of the atoms and molecules is
a fact, and not rather the very questionable result of an uncertain
method of calculation. Undoubtedly modern chemistry has established
the _relative_ weight-proportions of the atoms and molecules with
admirable precision, but it can make only very uncertain statements
in regard to the _absolute_ size of the ultimate particles. It is
therefore admissible to assume that these have a still greater degree
of minuteness when the facts in another domain of science require this.
We _must_ assume determinants, and consequently the germ-plasm must
have room for these; the variations of species can only be explained
through variations of the germ-plasm, for these alone give rise to
hereditary variation. It is upon this foundation that my germinal
selection is built up; whether I have in the main reached the truth the
future will show: but that I have not exhausted this new domain, but
only opened it up, I am very well aware.
LECTURE XXVII
THE BIOGENETIC LAW
Fritz Müller's ideas--Development of the Crustaceans--Of
the Daphnidæ--Of Sacculina--Of parasitic Copepods--Larvæ
of the higher Crustaceans--Change of phyletic stages in
Ontogeny--Haeckel's _Fundamental Biogenetic Law_--Palingenesis
and Cœnogenesis--Variation of phyletic forms by interpolation in
a lengthened Ontogeny--Justification of deductions from Ontogeny
to Phylogeny--Würtemberger's series of Ammonites--Phylogeny of the
markings in the caterpillars of the Sphingidæ--Condensation of
Phylogeny in Ontogeny--Example from the Crustaceans--Disappearance
of useless parts--The variation of homologous parts, according
to Emery--Germ-plasmic correlations--Harmony with the theory of
determinants--Multiplication of the determinants in the course of the
phylogeny.
What I propose to discuss in this lecture should have been considered
at an earlier stage, if we had pledged ourselves to adhere strictly
to the historical sequence of scientific discovery, for the phenomena
which we are about to deal with attained recognition shortly after
the revival of the evolution idea, and indeed they formed the first
important discovery which was made on the basis of the Darwinian
Doctrine of Descent. I have introduced them at this stage because
they have to do with phenomena of inheritance and modifications of
these, the understanding of which--in as far as we can as yet speak
of understanding at all--is only possible on the basis of a theory
of inheritance. Therefore, in order to examine these phenomena and
their causes, it was necessary first to submit a theory of heredity,
as I have done in the germ-plasm theory. We have to treat of the
connexion between the _development_ of many-celled individuals and
the _evolution_ of the species, between germinal history and racial
history, or, as we say with Haeckel, between ontogeny and phylogeny.
Long before Darwin's day individual naturalists had observed that
certain stages in the development of the higher vertebrates, such as
birds and mammals, showed a likeness to fishes, and they had spoken
of a fish-like stage of the bird-embryo. The 'Natural Philosophers'
of the beginning of the nineteenth century, Oken, Treviranus, Meckel,
and others, had, on the basis of the transmutation theory of the time,
gone much further, and had professed to recognize in the embryonic
history of Man, for example, a repetition of the different animal
stages, from polyp and worm up to insect and mollusc. But von Baer
afterwards showed that such resemblances are never between different
types, but only between representatives of the same general type,
e.g. that of Vertebrata; and Johannes Müller maintained, from the
standpoint of the old Creation theory, that an 'expression of the
most general and simple plan of the Vertebrates' recurred in the
development of higher Vertebrates, giving as an instance that, at a
certain stage of embryogenesis, even in Man, gill-arches were laid down
and were subsequently absorbed. But why this 'plan' should have been
carried out where it was afterwards to be departed from remained quite
unintelligible.
An answer to this question only became possible with the revival of
the Theory of Descent, and the first to throw light in this direction
was Fritz Müller, who, in his work _Für Darwin_, published in 1864,
interpreted the developmental history of the individual, 'the
ontogeny,' as a shortened and simplified repetition, a recapitulation,
so to speak, of the racial history of the species, the 'phylogeny.' But
at the same time he recognized quite clearly--what indeed was plain to
all eyes--that the 'racial history' cannot be simply read out of the
'germinal history,' but that the phylogeny is often 'blurred,' on the
one hand by the fusing and shortening of its stages, since development
is always 'striking out' a more direct course from the egg to the
perfect animal, while, on the other hand, it is frequently 'falsified'
by the struggle for existence which the free-living larvæ have to
maintain.
For the establishment of these views Fritz Müller relied chiefly upon
larvæ, and in particular upon those of Crustaceans, and the facts,
which were in part new and in part interpreted in a new manner, were
so striking that it was impossible to deny their importance. In
particular, he drew attention to the fact that in several of the lower
orders of Crustaceans the most diverse species have a similar form when
they leave the egg, all of them being small, unsegmented larvæ, with
a frontal eye and a helmet-like upper lip, and with three pairs of
appendages, the two posterior pairs being two-branched swimming-legs
beset with bristles. In the size and form of the body, and especially
of the chitinous carapace, these larvæ differ in the various systematic
groups; thus, for instance, the larvæ of the Copepods are simply
oval, while those of the Cirrhipedes are produced anteriorly into two
horn-like processes, and so on, but in essentials they are all alike,
and for a long time these larval forms had been distinguished by the
special name of 'Nauplius' (Fig. 109).
The development of the perfect animal begins with the longitudinal
growth of the Nauplius; the posterior end lengthens and becomes
segmented, between the anterior portion and the tail more segments are
interpolated, and on these new pairs of limbs may grow. The number
of these segments and limbs varies according to the group to which
the animal belongs. Thus the body of the perfect animal in the little
Cyprids always consists of eight segments, seven of which bear a pair
of limbs apiece; in the Branchiopods, on the other hand, the number
of segments varies from twenty to sixty, with ten to over forty pairs
of legs; in the Daphnids or water-fleas there are about ten segments,
with seven to ten pairs of limbs, and in the Copepods about seventeen
segments with eleven pairs of limbs. The difference between the orders
depends not only upon the differences in the number of segments and
limbs, but quite as much upon the form and development of the segments,
and above all of the limbs, and in this connexion it is worthy of note
that the additional limbs which grow out usually appear at first as
biramose swimming-legs, and are subsequently modified in form. Thus
the pairs of jaws, three in number, which appear in the Copepods are
developed from such swimming-legs, and so also is the second pair of
antennæ in the Copepods and the jaws of the Branchiopods, Cirrhipedes,
&c.
[Illustration: FIG. 108. Nauplius larva of one of the lower
Crustaceans. After Fritz Müller. _Au_, the frontal eye; _I_, first pair
of limbs, corresponding to the future antennæ; _II_ and _III_, two
biramose swimming appendages.]
If then we have before us in the 'germinal history' (ontogeny) a fairly
precise repetition of the 'racial history' (phylogeny), we may deduce
from this that the primitive forms of the Crustacean race were animals
which consisted of few segments, and that from these, in the course
of the earth's history, the very diverse modern groups of Crustaceans
have arisen, by the addition of new segments, and the adaptation of
the limbs upon them, which were at first biramose swimming-legs, to
different kinds of functions, one becoming an antenna, another a jaw
or a swimming-arm, a third, fourth, fifth, and so on, a jumping-leg, a
copulatory organ, an egg-bearer, a gill-bearer, or a tail-fin.
[Illustration: FIG. 109. Metamorphosis of one of the higher Crustacea,
a Shrimp (_Peneus potimirim_), after Fritz Müller. _A_, the nauplius
larva with the three pairs of appendages: _I_, the antennæ; _II_ and
_III_, the biramose swimming-feet. _Au_, the single eye. _B_, first
Zoæa stage, with six pairs of appendages (_I-VI_). _Skn_, area where
new segments are being formed.]
That the development has in general followed those lines is made clear
chiefly by the fact that the members of all these different orders of
Crustaceans still arise from nauplius larvæ, even in those cases in
which the perfect animal possesses a structure differing widely from
the usual Crustacean form. _All_ Crustaceans arise from the _nauplius
form_, even those of the higher orders, though they may not arise from
a nauplius _larva_. But this very circumstance, that in most of the
higher and many of the lower Crustaceans, the young animal, when it
emerges from the egg, already possesses more numerous segments and
limbs than a nauplius larva, again points to the connexion between
phylogeny and ontogeny, for in these cases the nauplius stage _is gone
through within the ovum_. The whole difference between this and the
forms we considered first lies in the fact that, in the latter, the
development is greatly shortened, condensed, as we might say, so that
the nauplius stage forms a part of the _embryonic_ development, and
that new segments and limbs develop in the embryo nauplius within the
egg, so that the young animal leaves the egg in a more advanced state,
nearer to that of the perfect animal, to which it can, therefore,
attain in a shorter time.
[Illustration: FIG. 109. _C_, second Zoæa stage. The thorax is now
divided into cephalothorax (_Cph_) and abdomen (_Abd_); seven pairs of
appendages are developed, and five more (_VIII-XII_) are beginning to
appear. _Au_, paired eyes.]
We should expect that this shortening of the larval period would be
associated with a prolongation of embryogenesis, especially in those
Crustaceans which possess a large number of segments and limbs, that
is--in the higher forms--and in the main this is the case. But there
are exceptions in two directions; in the first place there are some,
even among the lower Crustaceans, which leave the egg not as a nauplius
but in the perfect form of the adult, and secondly, there are, among
the higher Crustaceans, certain species which emerge from the egg not
in the more mature form but still in the primitive nauplius form.
Fritz Müller was the first to furnish an example of this last case,
a Brazilian shrimp, _Peneus potimirim_. Like the lowest Copepods or
Branchiopods, this species, which belongs to the highest order of
Crustaceans, goes through the whole long development, from the nauplius
through a series of higher larval forms up to the perfect animal,
and all _outside of the egg_, as an independent free-swimming larva
(Fig. 109, _A-E_). This is in sharp contrast to its near relative, the
freshwater crayfish, which goes through this whole development within
the egg, and emerges perfectly formed.
[Illustration: FIG. 109. _D_, Mysis-stage. Thirteen pairs of appendages
are now formed: _I_ and _II_, antennæ; _III_, mandibles; _IV_ and
_V_, maxillæ; _VI-XIII_, swimming appendages with one branch or with
two. _Abd_, abdomen. _Sfl_, tail-fin. _E_, the fully-formed Shrimp,
with thirteen pairs of appendages on the cephalothorax (_Cph_); _I_
and _II_, the two pairs of antennæ; then follow the maxillæ and
maxillipedes (_III-VIII_), the last of which is visible in the figure,
and the five pairs of walking-legs (_IX-XIII_) of which the third bears
a long chela. On the abdomen there are now six pairs of appendages
(_XIV-XIX_).]
We see from this example that it is not some inward necessity which
thus, in the higher and more complicated organism, contracts the
ontogeny into the embryonic state, but that this depends upon external
adaptive factors. Here again we have adaptation, mainly to the
conditions of larval life. The elimination of the larvæ by enemies,
for instance, will, other things being equal, be so much the more
incisive the longer the larval development is protracted, but in that
case the general ratio of elimination of the species, and the degree
of fertility the species must possess in order to hold its own in the
struggle for existence, will also play a part in determining the mode
of development. For the higher the ratio of elimination the more eggs
the female must produce, and the more eggs that have to be produced the
smaller will be the quantity of nutritive material for the building
up of the young embryo which each egg can be furnished with. I know
of no records in regard to the eggs of that Brazilian shrimp in which
embryonic development ends with the nauplius stage, but we shall
certainly not be wrong in predicting that the eggs in this case will be
very small and very numerous, in contrast to those of the freshwater
crayfish, which are large and, as compared with others known to us, not
very numerous.
It is a point of undeniable theoretical significance which the
life-histories of these Crustaceans disclose, that embryogenesis is
not condensed according to hidden internal laws when the structure
increases in complexity, but that the condensation of the ontogenetic
stages depends upon adaptation, and may be quite different in nearly
related species. It shows us anew that all biological occurrences are
dominated by the process of selection.