THE ORIGIN OF THE FORMS OF LIFE. 43 
arated by any great natural barrier, as a chain of lofty mount- 
ains or ‘an intervening body of water of considerable extent, 
and so prevented from breeding with forms that did not vary, 
it is clear that there would be greater likelihood of their differ- 
ences being preserved and augmented up to the point of their 
greatest usefulness. 
We may now inquire whether such has actually been the 
course of events in nature. The evidence may be arranged 
under the following heads: 
1. Morphology.—Briefiy, there is much that is common to 
entire large groups of animals; so great, indeed, are the resem- 
blances throughout the whole animal kingdom, that herein is 
found the strongest argument of all for the doctrine of descent. 
To illustrate by a single instance—fishes, reptiles, birds, and 
mammals possess in common a vertebral column bearing the 
same relationship to other parts of the animal. It is because of 
resemblances of this kind, as well as by their differences, that 
naturalists are enabled to classify animals. 
2. Embryology.—In the stages through which animals pass 
in their development from the ovum to the adult, it is to be ob- 
‘served that the closer the resemblance of the mature organism 
in different groups, the more the embryos resemble one another. 
Up to a certain stage of development the similarity between 
groups of animals, widely separated in their post-embryonic 
life, is marked: thus the embryo of a reptile, a bird, and a mam- 
mal have much in common in their earlier stages. The embryo 
of the mammal passes through stages which represent condi- 
- tions which are permanent in lower groups of animals, as for 
example that of the branchial arches, which are represented by 
the gills in fishes. It may be said that the developmental his- 
tory of the individual (ontogeny) is a brief recapitulation of 
the development of the species (phylogeny). Apart from the 
theory of descent, it does not seem possible to gather the true 
significance of such facts, which will become plainer after the 
study of the chapters on reproduction. 
3. Mimicry may be cited as an instance of useful adapta- 
tion. Thus, certain beetles resemble bees and wasps, which lat- 
ter are protected by stings. It is believed that such groups of 
beetles as these arose by a species of selection; those escaping 
enemies which chanced to resemble dreaded insects most, so 
that birds which were accustomed to prey on beetles, yet feared 
bees, would likewise avoid the mimicking forms. 
4. Rudimentary Organs.—Organs which were once functional 
