32 EVOLUTION 



another more closely in the early stages of their indi- 

 vidual development than they do in the adult condi- 

 tion, and in the earliest stages of all they are often 

 indistinguishable. These facts are explained if we 

 suppose that such individuals have a common origin, 

 that they are descended from a common ancestor, and 

 that traces of their pedigree are still to be observed in 

 the developmental stages through which each one 

 passes. We do not find a complete parallelism be- 

 tween the development of the individual and the 

 history of the race, nor should we expect to do so, 

 since embryonic as well as adult stages may be modified 

 in the course of evolution ; what we should expect is 

 a more or less vague historical sketch, and this is what 

 is usually found remaining. 



' It is not true, for example, that a fish is a reptile 

 arrested in its development, or that a reptile was ever 

 a fish ; but it is true that the reptile embryo, at one 

 stage of its development, is an organism which, if it 

 had an independent existence, must be classified 

 among fishes ; and all the organs of the reptile pass, in 

 the course of their development, through conditions 

 which are closely analogous to those which are per- 

 manent in some fishes/ 



3., MORPHOLOGY. On comparing together the dif- 

 ferent members of one of the great groups or classes 

 of animals or plants, we find the same fundamental 

 plan of organization running through all of them. 

 Series of corresponding organs are often to be made 

 out which are built upon the same general scheme * 

 although their functions may be quite dissimilar ; so 



