gg ANIMAL LIFE 



hopper goes through the same developmental changes from 

 single egg cell to the full-grown active hopper as every 

 other grasshopper of the same kind that is, development 

 takes place according to certain natural laws, the laws of 

 animal development. These laws may be roughly stated as 

 follows : All many-celled animals begin life as a single cell, 

 the fertilized egg cell ; each animal goes through a certain 

 orderly series of developmental changes which, accom- 

 panied by growth, leads the animal to change from single 

 cell to the many-celled, complex form characteristic of the 

 species to which the animal belongs ; this development is 

 from simple to complex structural condition ; the develop- 

 ment is the same for all individuals of one species. While 

 all animals begin development similarly, the course of devel- 

 opment in the different groups soon diverges, the diver- 

 gence being of the nature of a branching, like that shown 

 in the growth of a tree. In the free tips of the smallest 

 branches we have represented the various species of ani- 

 mals in their fully developed condition, all standing clearly 

 apart from each other. But in tracing back the develop- 

 ment of any kind of animal, we soon come to a point where 

 it very much resembles or becomes apparently identical 

 with some other kind of animal, and going further back we 

 find it resembling other animals in their young condition, 

 and so on until we come to that first stage of development, 

 that trunk stage, where all animals are structurally alike. 

 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 some- 

 thing inherent in each developing animal that gives it an 

 identity of its own. Although 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. 

 The young fish and the young salamander in the upper row 

 in Fig. 41 seem very much alike, but one embryo is sure to 



