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 
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, accompanied by growth 
leads the animal to change from a single cell to the many-celled, 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 same 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 fully 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 
