142 EVOLUTION OF TO-DAY. 



as a fact but do not understand, is an explanation 

 of this parallel. With this principle we can under- 

 stand why it is that the embryos of higher animals 

 resemble the adult stages of lower ones, and re- 

 semble also the animals of geological times; we 

 see why it is that embryology is always an attempt 

 at a repetition of past history, and also why it is 

 that, owing to surrounding conditions, the attempt 

 is not always successful. It has been the work of 

 the last twenty years of embryology to show that 

 this explanation is satisfactory, and that it throws 

 light upon the whole subject of animal life. Al- 

 though, as we shall presently see, many questions 

 still remain for solution, enough has been done to 

 show that the result will eventually be a harmony. 



This is the simplest explanation for the fact 

 offered, but it is not a conclusion that cannot be 

 gainsaid. Many people have fully recognized the 

 parallel in question and yet denied evolution. Just 

 as it is possible to claim that, while the facts of 

 paleontology are such as would follow from the 

 theory of evolution, they simply prove that animals 

 of higher and higher organization have appeared in 

 successive ages, and do not prove genetic descent ; 

 just as it is possible to claim that, while all of the 

 facts of classification and homology receive their 

 most simple explanation from the descent theory, 

 yet they do not prove it : so here it is possible to 

 contend that this remarkable fact is only a part of 

 a preconceived harmony a proof of the unity of 

 the organic world. And for this unity we have seen 

 that the theory of types offers its explanation. The 



