726 Growth and Development. [ July, 
adapted to simpler or to more complex conditions of nature, and 
the question as to whether a creature is higher or lower in rank 
depends entirely upon the degree of complexity in its adap- 
tations. 
Embryonic development closely follows the ancestral line. If 
there has been a retrogression, the point from which the fall com- 
menced is always attained by the larva, as in the case of the 
barnacles and in other instances. But the successive changes of 
condition are not all clearly displayed. Some stages of develop- 
ment are retarded, others hurried through. It is probably a ques 
tion of the influence of external conditions. Of the conditions 
of nature to which the various ancestral forms of the animal 
were adapted, many have vanished. Some yet exist. Thus in 
some stages of larval life the animal would find no support from 
nature. In others it is adapted to existing nature. The former 
stages are hurried or slurred over in development, the latter are 
passed through slowly. Of the many thousands of ancestral 
forms which the embryo might exhibit, the great mass succeed 
and overlap each other so rapidly as to be indistinguishable, 
while some persist as marked conditions of larval life. 
And if the animal is forcibly retained under conditions favor- 
able to one of its larval phases of development, its individual life 
may long continue in that phase, as in the cases above cited. 428 
lives of intestinal parasites present marked instances of this kind. 
One phase of life is pursued for an indefinite period in one host. 
Yet as soon as another host is entered, and the animal exposed 
to new contact influences, and surrounded by new conditions, 
growth is succeeded by development, and a new life phase a5- 
sumed. One instance of this is that of the Trichinia, which lays 
its eggs only in the intestine of its second host. 
It would seem as if the conditions surrounding the larva 
strongly favored growth in that life stage, and hindered the m- 
nate tendencies to develop. For the latter to come fully into 
play, the animal must enter into the conditions necessary to & 
next life stage, or at least be withdrawn from active external m 
fluence, so as to permit the play of organic chemistry within its 
tissues, and the consequent unfoldment of new conditions 0! the 
tissues. : 
The facts of insect transformation present the most striking 
instances of the life process above considered. In the higher 
