1 24 E VOL UTION OF TO-DA Y. 



He plainly saw the principle, and used it as an argu- 

 ment in his essay on classification. The embryo 

 everywhere begins as a simple undifferentiated 

 organism, and builds itself up through successively 

 higher and higher grades, until it finally reaches the 

 complicated adult. And so in fossil history it ap- 

 pears, rather indefinitely and yet distinctly, that 

 the lower forms appear in the earlier ages, and give 

 place as the ages go by to successively higher and 

 higher forms, until the most complicated appear 

 last. But such a general parallelism as this has no 

 meaning, and, unless more definite evidence can be 

 found, our assumption is of no importance. 



But if we cease to make the question quite so 

 broad and confine it to special cases, we may per- 

 haps get more definite conclusions. And here we 

 labor under the disadvantage that, for reasons that 

 we shall see later, the vertebrates which have the 

 most complete fossil history have the most incom- 

 plete embryological history. The only systematic 

 attempt at such a comparison as we are considering, 

 has been made by A. Agassiz. Having at his com- 

 mand a great number of fossil echinoids, he has 

 made a very careful study of them for the purpose 

 of discovering how accurately a parallel could be 

 drawn between their history and that of the young 

 echinoid. The result of this comparison was to sub- 

 stantiate the theory in its general bearings. " We 

 are justified," he says, " in seeking for our earliest 

 representative of the order such echinoderms as 

 resemble early stages of our embryos." But he con- 

 cludes further that " any thing beyond a general 



