232 



EVOLUTION AND ANIMAL LIFE 



fully developed, mature stages of lower animals. To be sure, 

 any animal at any stage in its existence differs absolutely from 

 any other kind of animal, in that it can develop into only its 

 own kind of animal. There is something inherent in each 

 developing animal that gives it an identity of its own. Al- 

 though in its young stages it may be hardly distinguishable 

 from some other kind of animal in similar stages, it is sure to 

 come out, when fully developed, an individual of the same 

 kind as its parents were or are. A very young fish and a very 

 young salamander are almost indistinguishably alike, but one 

 is sure to develop into a fish and the other into a salamander. 

 This certainty of an embryo to become an individual of a 

 certain kind is called the law of heredity. Viewed 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 the developmental phenomena is a 

 matter about which naturalists have yet very much to learn. 



FIG. 135. Stages in the development of the prawn, Peneus potimirium : A, Nauplius 

 larva; B, first zoea stage; C, second zoea stage. (After Fritz Miiller.) 



It is believed, however, by practically all naturalists that many 

 of the various stages in the development of an animal cor- 

 respond to or repeat, in many fundamental features at least, 

 the structural condition of the animal's ancestors. Naturalists 

 believe that all backboned or vertebrate animals are related 

 to each other through being descended from a common ancestor, 

 the first or oldest backboned animrJ. In fact, it is because all 



