THE PROBLEM: THE MODE OE ITS SOLUTION 23 



Then the zoologist began to ask and investigate how 

 the animal grew in the egg and attained its definite 

 form. And this study of embryology brought to light 

 many new and interesting facts. Agassiz especially 

 emphasized and maintained the universality of the fact 

 that there was a remarkable parallelism between em- 

 bryos of later forms and adults of old or fossil groups. 

 The embryos of higher forms, he said, pass through and 

 beyond certain stages of structure, which are permanent 

 in lower and older members of the same group. 



You remember that the fin on the tail of a fish is as 

 a rule bilobed. Now the backbone of a perch or cod 

 ends at a point in the end of the tail opposite the angle 

 between the two lobes, without extending out into either 

 of them. In the shark it extends almost to the end of 

 the upper lobe. Now we have seen that sharks and 

 ganoids are older than cod. In the embryo of the cod 

 or perch the backbone has, at an early stage, the same 

 position as in the shark or ganoid; only at a later 

 stage does it attain its definite position. 



So Agassiz says the young lepidosteus (a ganoid 

 fish), long after it is hatched, exhibits in the form of its 

 tail characters thus far known only among the fossil 

 fishes of the Devonian period. The embryology of 

 turtles throws light upon the fossil chelonians. It is 

 already known that the embryonic changes of frogs 

 and toads coincide with what is known of their succes- 

 sion in past ages. The characteristics of extinct genera 

 of mammals exhibit everywhere indications that their 

 living representatives in early life resemble them more 

 than they do their own parents. A minute comparison 

 of a young elephant with any mastodon will show this 

 most fuUy, not only in the peculiarities of their teeth, 



