The present work by Professor Weismann, well known for his profound
embryological investigations on the Diptera, will appear, I believe, to
every naturalist extremely interesting and well deserving of careful
study. Any one looking at the longitudinal and oblique stripes, often
of various and bright colours, on the caterpillars of Sphinx-moths,
would naturally be inclined to doubt whether these could be of the
least use to the insect; in the olden time they would have been called
freaks of Nature. But the present book shows that in most cases the
colouring can hardly fail to be of high importance as a protection.
This indeed was proved experimentally in one of the most curious
instances described, in which the thickened anterior end of the
caterpillar bears two large ocelli or eye-like spots, which give to
the creature so formidable an appearance that birds were frightened
away. But the mere explanation of the colouring of these caterpillars
is but a very small part of the merit of the work. This mainly consists
in the light thrown on the laws of variation and of inheritance by
the facts given and discussed. There is also a valuable discussion
on classification, as founded on characters displayed at different
ages by animals belonging to the same group. Several distinguished
naturalists maintain with much confidence that organic beings tend to
vary and to rise in the scale, independently of the conditions to which
they and their progenitors have been exposed; whilst others maintain
that all variation is due to such exposure, though the manner in which
the environment acts is as yet quite unknown. At the present time
there is hardly any question in biology of more importance than this
of the nature and causes of variability, and the reader will find in
the present work an able discussion on the whole subject, which will
probably lead him to pause before he admits the existence of an innate
tendency to perfectibility. Finally, whoever compares the discussions
in this volume with those published twenty years ago on any branch of
Natural History, will see how wide and rich a field for study has been
opened up through the principle of Evolution; and such fields, without
the light shed on them by this principle, would for long or for ever
have remained barren.