Chap. I. GROWTH OF TURTLES. 291 



which I have not soon in its A-oungost state. In the Trionjohidio, tliis flat, orbic- 

 ular form is preserved through lilb, and in the Euiydoidoe during the first four 

 or five years, at least; but by and ])y the shield assumes the more or less ellip- 

 tical and higher form of the adult, according to the different genera and species. 

 This change takes place earlier in the Chelydroida? and CinosternoidEC than in the 

 Emydoidiv, and earlier still in the Chelonioida;.^ In this last family, the character- 

 istic features of the adult are already sketched out in the first year, though not 

 yet fully developed. Tn the fixmily of Chelydroidas, the embryonic characters are 

 prevalent for two years at least; in that of Cinosternoidoe the characters of the 

 young do not disappear befoi-e the fourth year. It is nevertheless true that each 

 family has its special pattern. 



The young Turtles are mostly so different from the adult, in all their features, 

 that it is very dilficult to identify them. At all events, it requires a long experi- 

 ence to recognize them, in these first years, for what they are. Our systematic 

 works, even the most recent, furnish, in fiict, the painful evidence that these young 

 Turtles have repeatedly been mistaken f(jr distinct species. On the other hand, 

 it is worth mentioning, that Turtles belonging to the same genus, as the genera 

 are circumscribed below, show already in the youngest state slight peculiarities 

 which at least indicate the genus, though the generic characters are by no 

 means all developed. In the family of the Emydoidtv, I have further observed 

 that the young approximate the lower Testudinata. not only by their i-emarkable 

 similarity with the Chelonioidce in the earlier stages of their embryonic develop- 

 ment, but also by their mode of life, which is much more aquatic than th;it of 

 the adult of the same species. This agrees remarkably well with the law, which 

 seems to exist throughout the animal kingdom, that aquatic animals rank lower 

 than the terrestrial representatives of the same groups.^ It may be remembered 

 in this connection, that in a large number of Insects the larvse live in the water, 

 while the perfect Insects are entirely aerial. Still greater differences, in the mode 

 of life and the form of the young and adults, may be observed among parasitic 

 Worms. Among Vertebrates, similar diflerences are jiarticularly obvious in the 

 class of Batrachians, in which the young of some of their representatives are 

 entirely aquatic, whilst the adults live exclusively upon land. At least, this is 

 the case for the highest among them, the Toads. These i-emarks in relation to 

 the development of the form, and the mode of life, which is more or less con- 

 nected with the form, may be sufficient to show how important the study of 

 young animals is with reference to a correct appreciation of their true relations. 



The following table gives a complete view of the changes which our common 

 Chrysemys undergoes in its form. 



* See Part III. for further details. ^ Comiwre Tart I., Sect. 9, p. 30. 



