1883. | Growth and Development. 727 
animals, indeed, the conditions of embryonal life preclude the 
long retention of larval stages. The embryo here is fully devel- 
oped within the body of the parent, or within the egg with its 
proper conditions of warmth and nutriment. There is no hin- 
drance to a rapid development. But in many of the lower tribes 
the young is born and abandoned to the influences of outer 
nature while still in an early stage of embryonal growth. In its 
further process of development it must be exposed in some stages 
to advantageous, in others to disadvantageous conditions. Natural 
selection will act to lengthen the period of the former, and 
shorten that of the latter. The animal will develop irregularly, 
now remaining long in one phase, now hurrying rapidly through 
several successive phases. And the retention of any one phase 
of life is not simply an effect of natural selection, but also of the 
principle above enunciated, that the action of favoring external 
contacts tends to restrain the operation of the innate tendencies 
to development, and to promote simple nutrition and growth 
without change of organs. 
In insect larvæ very active nutrition takes place. The tissues 
increase rapidly in size, but their further development is, for the 
time, arrested. Other important effects result. The animal whose 
life is arrested at the larval stage being exposed to all the mold- 
ing influences of nature, gains specific variations similar to those 
which occur in mature animals. As the conditions to which the 
larva was originally adapted change, it changes in accordance. 
It gains special habits and organs necessary to its success in this 
stage of life, yet forming no part of its native plan of develop- 
ment. These are adventitious organs, and are thrown off by’ 
the animal in its pupal development as useless additions to 
the body. But the most marked and singular instance of this 
Principle of growth occurs in another branch of the animal. king- 
dom, the Echinodermata. There is nothing more remarkable in 
the history of animal transformations than that displayed in the 
development of the various members of the Echinoderm races. 
Yet these strange transformations are undoubtedly results of the 
Principle of development here enunciated. Only the core of the 
is indicated in the form of the swimming larva. It has 
gained many adventitious organs, probably as results of a long 
Process of adaptation to conditions surrounding its larval life, but 
Which are utterly outside its original life plan. Only the deep- 
