EVIDENCES FROM EMBRYOLOGY 169 



is readily seen. All rabbits develop in the same way; every grass- 

 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 

 roughjy stated as follows: All many-celled animals begin life as a 

 singlej:ell, the fertilized egg cell; each animal goes through a certain 

 orderly series of developmental changes which, accompanied by growth 

 leads the animal to change from a single cell to the manyj^elled, com- 

 plex form characteristic of the species to which the animal belongs; 

 this development is from simple to complex structural condition; the 

 development is the saijie for all individuals of one species. While all 

 animals begin development similarly, the course of development in 

 the different groups soon diverges, the divergence 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 animals in their !ully developed condition, all standing more or less 

 clearly apart from each other. But in tracing back the development 

 of any kind of animal we soon come to a point where it very much 

 resembles or becomes apparently identical with the development of 

 some other kind of animal, and, in addition, the stages passed through 

 in the developmental course may very much resemble the fully devel- 

 oped, 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. Although in its young stages it may be hardly distin- 

 guishable 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 sala- 

 mander 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. It is believed, how- 

 ever, by practically all naturalists that many of the various stages in 

 the development of an animal correspond to or repeat, in many 



