14 VERTEBRATE ZOOLOGY 



types, including the dinosaurs, ichthyosaurs, plesiosaurs, and ptero- 

 saurs, all of which became extinct before the close of the Mesozoic. 

 Only a few of the more generalized stocks lived over into the Cenozoic 

 to found the rather conservative modern reptilian classes. 



The generalized forms are like the conservative germ-plasm in 

 heredity, that passes on from generation to generation, progressing 

 slowly and steadily along definitely directed lines, and largely un- 

 influenced by the somatic specializations that are the result of en- 

 vironmental or functional adaptations. Just as the germ-plasm is a 

 continuous, unbroken series of cell generations that gives off tangen- 

 tially the successive somatic generations with all of the accompany- 

 ing specializations, so is the generalized stock of animal forms a con- 

 tinuous series of races that sprays off at intervals the specialized 

 types. These specialized groups go their ways and become extinct, 

 while the slowly evolving generalized types form the stock from 

 which future specialized races may arise. 



Generalized forms are never equally generalized in all particulars. 

 As a rule, while retaining a fundamentally generalized structure, they 

 become superficially specialized in one or more particulars. While 

 the ideally generalized type is only approximated in any class of 

 vertebrates, some one or more forms stand out from their fellows as 

 more nearly prototypic than the rest. Such forms are of great phylo- 

 genetic interest and will subsequently claim a large share of our atten- 

 tion. 



Not infrequently forms that appear to have some of the ear-marks 

 of prototypes prove on examination to be pretenders, in that they are 

 either secondarily simplified by parasitic or sedentary life, or are 

 retarded forms that have failed to complete the life cycle normal for 

 the group to which they belong. Usually the degenerate forms may 

 readily be recognized by the study of their life history; for their larval 

 stages show more advanced conditions of various structures than do 

 the definitive forms. The retarded forms, on the other hand, simply 

 cease to develop somatically in a larval or juvenile stage, while the 

 germinal cycle completes itself. As a consequence these juvenile 

 forms become sexually mature and reproduce their kind, a phenom- 

 enon known as pcedogenesis or neoteny. 



Specialized types should be carefully discriminated from senescent 

 types, though few specialized forms are free from the evidences of 

 senescence. Specializations are to be looked upon as adaptive in 



