134 Embryology and Growth of Fishes 



ing animal that gives it an identity of its own. Although in its 

 young stages it may be indistinguishable from some other species 

 of animal in its young stages, it is sure to come out, when fully 

 developed, an individual of the same kind as its parents were or 

 are. The voung fish and the young salamander may be alike 

 to all appearance, but one embryo is sure to develop into a 

 fish, and the other into a salamander. This certainty of an 

 embrvo to become an individual of a certain kind is called the 

 law of heredity. \Tewed in the light of development, there 

 must be as great a difference between one egg and another as 

 between one animal and another, for the greater difference is 

 included in the less. 



The Significance of Facts of Development. — The significance 

 of the process of development in any species is yet far from com- 

 pletely understood. It is believed that many of the various 

 stages in the development of an animal correspond to or repeat 

 the structural condition of the animal's ancestors. Naturalists 

 believe that all animals having a notochord at any stage in 

 their existence are related to each other through being descended 

 from a common ancestor, the first or oldest chordate or back- 

 boned animal. In fact it is because all these chordate animals — 

 the lancelets, lampreys, fishes, batrachians, the reptiles, the birds, 

 and the mammals — have descended from a common ancestor that 

 they all develop a notochord, and those most highly organized re- 

 place this by a complete back-bone. It is believed that the de- 

 scendants of the first back-boned animal have, in the course of 

 many generations, branched off little by little from the original 

 type until there came to exist very real and obvious differences 

 among the back-boned animals — difi:erences which among the liv- 

 ing back-boned animals are familiar to all of us. The course of 

 development of an individual animal is believed to be a verv 

 rapid and evidently much condensed and changed recapitula- 

 tion of tlie history which the species or kind of animal to which 

 the developing individual belongs has passed through in the 

 course of its descent through a long series of gradually changing 

 ancestors. If this is true, then we can readily understand why 

 the fish and the salamander and the tortoise and bird and rabbit 

 are all alike in their eariier stages of development, and gradually 



