730 Growth and Development. [July, 
these specific organs appear, to be secondarily modified through, 
the influence of changed conditions of nature. 
The question might here be reasonably asked, why, if the lar- 
val condition of the insect is often so superior to the imago for 
purposes of nutrition, did the animal ever advance to a more in- 
tricate life stage? Why did it not persist in its better adapted 
ancestral form. This question it may not be difficult to answer. 
There have been very great changes. in natural conditions, and 
the relations of insect life have varied accordingly. Insects were, 
in all probability, the first flying animals. If so, the possession 
of powers of flight was a highly advantageous condition. It en- 
abled the original insects to escape from their enemies on the land, 
and they had no foes in the air. At this period, then, there was 
probably no retardation in the larval stage, and the imago stage 
may have long continued. Such a condition persists in some 
species of insects. Later, however, the air became the home of 
other flying animals, and the insect lost the security which it had 
previously enjoyed. In the weaker and more exposed tribes, nat- 
ural selection produced a lessening of the length of the imago 
life period, and a hastening of the reproductive activity. But as 
mature life was checked, larval life was lengthened. A certain de- 
gree of nutrition was necessary, and could now be more safely 
attained in the larval stage. 
The same variation of conditions may have acted to produce 
the larval retardation of the Crustacea, the Echinodermata and 
other tribes. The soft-bodied and helpless Medusa seems partici- 
larly subject to danger from foes. In its original development it 
may have been much less so. A subsequent rapid destruction of 
the mature animals may have caused the development of the bet- 
ter protected colony of Hydrozoan larve. 
Some final consideration of the method of 
changes seems desirable. There are inherent in the ge 
gies and tendencies, chemical, molecular, or whatever we is 
to call them, adapted to the complete unfoldment of the Be 
form. But, as appears evident, their operation can be checked ue 
influences from external nature. There is a struggle w i 
these contact influences and the innate organic energies. m i 
ter are the resultants of numerous previous contacts Wi | ts | 
acted on the whole ancestral line of the animal. The mind, ge ne 
inherited tendencies, represents these ante-natal forces. The action 
developmental 
rm ener- 
hich have | 
