88 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 
