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THE PRINCIPLES OF BIOLOGY. 
THE PRINCIPLES OF BIOLOGY. 
BY HERBERT SPENCER. 
Part V.— Physiological Development. Chapters YI. to X. 
These were expounded by Professor Haycraft, F.R.S.E., 
then Professor of Physiology at the Mason College. The 
Professor gave a detailed explanation of the course of the 
Physiological Development of Animals, as narrated by Mr. 
Herbert Spencer, and increased the interest of the narrative 
by many apt and striking illustrations. 
Part YI. —The Laws of Multiplication. Chapters I. to VIII. 
BY W. B. GROVE, B.A. 
“ If organisms have been evolved, their respective powers 
of multiplication must have been determined by natural 
causes.” The adaptation of the reproductive activity of an 
organism to its conditions of existence has been determined 
by the same means—the action and reaction between it and 
its environment—by which its other adaptations have been 
effected. 
We speak here only of organisms which maintain their 
ground. Those species which are dying out are excluded 
from the present argument. If a species is flourishing, it is 
so because of a certain relation established between the 
forces which tend to preserve it and the forces which tend to 
destroy it; and Mr. Spencer’s enquiry is directed to the 
purpose of ascertaining if this relation is capable of being 
expressed in a definite numerical form—as indeed the laws 
of multiplication should be. 
If the forces destructive of race, when once in excess, 
had nothing to prevent them from remaining in excess, the 
race would disappear; and if the forces preservative of race, 
when once in excess, had nothing to prevent them from 
remaining in excess, the race would go on increasing to 
infinity. There must be some way by which the excess of 
either of the conflicting forces is automatically adjusted; 
for the only alternative is to call in that perpetual meddling 
with the world on the part of its Creator which the older 
theologies postulated, but which hardly anyone would now 
seriously maintain. 
There must thus be here, as wherever antagonistic forces 
are in action in the world, a rhythmic movement, an alternate 
predominance of each, and each by its very excess must call 
