1895.] Organic Variation. — 893 
ontogenetically by the parent. Continuous nutrition, and, 
apparently, also continuous bodily activity, act to check this 
process of development, which appears to need cessation of the 
assimilative process and of physical or nervous activity, all 
the organic powers being concentrated upon the event about 
to take place. 
Nor is this all that may be necessary. Stimulation from 
without seems often requisite to start the developmental pro- 
cess. Stimulation from within is perhaps equally necessary, a 
psychic influence it may be, arising in the inherent instinets of 
the central ganglion of the nervous system. External stimu- 
lation may, in some cases, be necessary to set these instincts in 
action, while in other cases, they may act involuntarily at a 
certain stage of ganglionic growth or development. It is ap- 
parently due to such influences of instinct, that nutrition is 
checked and the inherent tendencies to changes in the tissues 
are permitted to act, the action of instinct being thus perhaps 
secondary ; though it may be that a direct stimulation from the 
ganglion to the tissues is necessary to set the powers of develop- 
ment in operation. The action of the mental powers may, there- 
fore, be confined to checking nutrition and activity, but may 
also concentrate the physical energies upon the region of coming 
change, and set in train the necessary chemical action. All 
the further powers and tendencies requisite exist in the tissues 
themselves. 
We possess abundant evidence that, in the lower animals, 
development will not proceed if the surrounding conditions be 
unfavorable, whatever be the inherent tendencies. The life-his- 
tory of intestinal parasites furnishes marked examples of this. 
Such creatures may continue a larval existence for an indefi- 
nite period in one host, the development to the mature stage 
being accomplished only after the second host is entered. 
Possibly, in the first host, nutrition continues active, and is 
checked on reaching the second host; but the influence of the 
new environment may have its special stimulating effect. The 
development of insects present many cases in point. They 
often continue long in the larval state, in which nutrition is 
active, growth rapid, and development checked. Then, during 
