THEORETICAL BIOLOGY 473 



cells forming a sphere; slightly higher up an individual con- 

 sists of a .sac whose wall is made up of two layers ol cells. 

 Now all animals above the Protozoa pass through these three 

 stages in the course of their development: first the single cell, 

 the fertilized ovum; then the sphere of cells of which the 

 blastula is a type; then the two-layered sac represented by 

 the gastrula. In other words, each individual in its earlier 

 development passes through stages represented by the simplest 

 animals. The embryos of the various classes of vertebrates so 

 closely resemble each other at a very early stage (A develop- 

 ment that it is well-nigh impossible to tell them apart ; young 

 Mammalia cannot be distinguished from one another. All the 

 vertebrates at early stages of development are strikingly like 

 embryonic fishes in structure ; like them they have gill slits and 

 the corresponding structure of heart and arrangement of blood 

 vessels; as development goes on, the special characteristics ol 

 each class appear. In the case of the birds and the mammals 

 certain amphibian and reptilian characteristics succeed one 

 another; finally the distinctive structures of the bird and the 

 mammal appear, and the peculiar features of the order, family, 

 genus, and species develop. Thus in the development of the 

 higher animals there is a tendency to repeat stages common to 

 lower groups, a fact which strongly indicates a descent from 

 such ancestral types. All intermediate steps are not neces- 

 sarily represented, but the general rule holds good and consti- 

 tutes what the German scientist, Haeckel, has called the 

 biogenetic law, that ontogeny is a recapitulation of phylogeny ; 

 in other words, that the development of the individual is a 

 repetition of the development of the race. 



Embryology furnishes still further evidence for evolution in 

 the presence of numerous rudimentary structures which never 

 appear in adult life. Thus teeth are found in some embryonic 

 turtles, although they occur in no adult Chelonia. Some birds, 

 such as parrots, have teeth in the embryo, although no adult 

 living bird has teeth. The embryos of the whalebone whales 

 likewise have teeth, although none are present in the adult. 

 These facts all point to the conclusion that such types h 

 descended from ancestors in which teeth were functional. 



One other instance of embryological evidence is of interest; 



