GENERAL BIOLOGY, 45 



loBt. The membranes aii4 appendages of the embryonic body (the amnion, yellc- 

 Bac, allantois) are omitted. The whole twelve figures are slightly magnified^ the 

 upper ones more than the lower. To facilitate the comparison, they are all re- 

 duced to nearly the same size in the cuts. All the embryos are seen from the left 

 side ; the head extremity is above, the tail extremity below ; the arched back 

 turned to the right The letters indicate the same parts in all the twelve flguree, 

 namely: v, fore-brain; z, twixt-brain; m, mid-brain; A, hind-brain; n, after-Brain; 

 r, spinal marrow; e, nose; a, eye; o, ear; A, gill-arches; g, heart; w, vertebral 

 column; /, fore-limbs; 6, hind-limbs,; «, tail. (After Haeckel.) 



If a certain proportion of forms varying similarly were sep- 

 arated by any great natural barrier, as a chain of lofty mountains 

 or an intervening body of water of considerable extent, and so pre- 

 vented from breeding with forms that did not vary, it is clear that 

 there would be greater likelihood of their di£Feren@es being pre- 

 served 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 un- 

 der the following heads : 



1. Morphology. — Briefly, there is much that is common to 

 entire lai'ge 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. 



3. 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. 

 'yi0to 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 a;rches, 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 signifi- 

 cance 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 adaptation. 



