Another of the great steps in organic evolution was the differentiation
of two different physiological types, the male or sperm-producer and the
female or egg-producer. It seems to be a deep-seated difference in
constitution, which leads one egg to develop into a male, and another,
lying beside it in the nest, into a female. In the case of pigeons it
seems almost certain, from the work of Professor Oscar Riddle, that
there are two kinds of egg, a male-producing egg and a female-producing
egg, which differ in their yolk-forming and other physiological
characters.
In sea-urchins we often find two creatures superficially
indistinguishable, but the one is a female with large ovaries and the
other is a male with equally large testes. Here the physiological
difference does not affect the body as a whole, but the reproductive
organs or gonads only, though more intimate physiology would doubtless
discover differences in the blood or in the chemical routine
(metabolism). In a large number of cases, however, there are marked
superficial differences between the sexes, and everyone is familiar with
such contrasts as peacock and peahen, stag and hind. In such cases the
physiological difference between the sperm-producer and the
ovum-producer, for this is the essential difference, saturates through
the body and expresses itself in masculine and feminine structures and
modes of behaviour. The expression of the masculine and feminine
characters is in some cases under the control of hormones or chemical
messengers which are carried by the blood from the reproductive organs
throughout the body, and pull the trigger which brings about the
development of an antler or a wattle or a decorative plume or a capacity
for vocal and saltatory display. In some cases it is certain that the
female carries in a latent state the masculine features, but these are
kept from expressing themselves by other chemical messengers from the
ovary. Of these chemical messengers more must be said later on.
Recent research has shown that while the difference between male and
female is very deep-rooted, corresponding to a difference in gearing, it
is not always clear-cut. Thus a hen-pigeon may be very masculine, and a
cock-pigeon very feminine. The difference is in degree, not in kind.
§ 5
What is the meaning of the universal or almost universal inevitableness
of death? A Sequoia or "Big Tree" of California has been known to live
for over two thousand years, but eventually it died. A centenarian
tortoise has been known, and a sea-anemone sixty years of age; but
eventually they die. What is the meaning of this apparently inevitable
stoppage of bodily life?