EVIDENCES— EXISTING ORGANISMS 109 



than that of the forms of the one variable species, and so, step by- 

 step, the varying degrees of relationship made evident by our 

 classification are evidences of more and more remote common 

 sources. 



Ontogenetic Succession. For an illustration of the succession 

 of changes which may have passed in the development of existing 

 forms, it is possible to look to the actual record, which is now 

 availa])le in sufficient extent to be here treated in(lei)endently. 

 But the things to which man first had to turn in his attempts to 

 explain the relationship of living creatures are entirely within the 

 organism. 



In studying the common characters of the vertebrates we have 

 noted the succession of forms characteristic of the several phyla. 

 That these forms may be looked upon as a chronological succession, 

 and not merely a succession in degrees of complexity is made 

 evident by the combination of anatomical and embryological 

 facts. In the skeleton of vertebrates, for example, the occur- 

 rence in some fishes of a cartilaginous cranium alone, in others of 

 such a cranium partly ossified together with external bony plates 

 with characteristic arrangement, and in the higher classes of a 

 skull which embryology shows to be made up of a similar cartilag- 

 inous portion in the beginning, from which certain bones are 

 derived by ossification and to which others are added by develop- 

 ment directly from mesodermal tissue, is indicative of relation- 

 ship in chronological succession. The same applies to other bones 

 of the body. This gradual succession of stages during embryo- 

 logical development which correspond to those represented by 

 adults of the several classes is clear evidence that the higher forms 

 have come from lower in this phylum. We see in the few points 

 mentioned that the formation of cartilage is not an essential step 

 in the formation of bone, so the transition of some bones can mean 

 only that they still pass through the stages which they have 

 followed in the past. 



The circulatory system, especially in the development of the 

 aortic arches, is a similar case. While they are symmetrically 

 paired in the fishes, amphibia and reptiles, although reduced in 

 number in the last two, they are still further reduced and asym- 

 metrically developed in the birds and mammals. Superficially the 

 resemblance is slight, but when we consider the embryological 

 succession of parts, and see even in the human embryo the develop- 



