820 On Certain Factors of Evolution. 
with the work of those who have brought together certain facts 
bearing on the physical factors of evolution.! The principal fac- 
tors referred to by Mr. Spencer are use and disuse and the influence 
of light. In one place he does in concrete language sum up these 
agencies as follows :— 
“The growth of a thing is effected by the joint operation of 
certain forces on certain materials; and when it dwindles there is 
either a lack of some materials or the forces co-operate in a way 
different from that which produces growth. ... . That is to say, 
growth, variation, survival, death, if they are to be reduced to the 
forms in which physical science can recognize them, must be 
expressed as effects of agencies definitely conceived—mechanical 
forces—light, heat, chemical affinity, etc.” (pp. 39, 40). 
On page 70 Mr. Spencer remarks :— 
“ But nevertheless, as we here see, natural selection could operate 
only under subjection. It could do no more than take advantage 
of those structural changes which the medium and its contents 
initiated.” 
Again, on page 73, Spencer suggests that natural selection, 1m 
order to act, must have had a limited number of organisms upon 
which to operate? As he remarks :— : 
“Though natural selection must have become increasingly active 
when once it had got a start, yet the differentiating action of the 
medium never ceased to be a co-operator in the development of 
these first animals and plants.” 
: In the writer’s Introduction to the Standard Natural History, 1885, 
under the head of Evolution (pp. 1 and Ixii.), he has endeavored to bring 
together references to the different authors who have insisted on ee 
which are in the line of those first suggested by Lamarck, @ phase : 
evolution which we have called Neolamarckianism. The aathors e 
Europe, Semper, Kölliker, Wagner, Martins, Plateau, Weismann, and 
rn, and in this country Haldeman, Leidy, Wyman, Clark, Cope 
Hyatt, Walsh, Allen, W. H. Edwards, Dall, and the writer. ee 
2 This point is one which the writer has also made and pon a A 
over twelve years ago in a communication to the Nation, holding 
it is an important objection to the theory of natural selection, the aed 
nature of which involves the existence of a world already stocked W m 
life forms. What the theory of evolution should explain is the pcm 
these first ordinal and class forms. Given even a scanty fauna, isore ia 
members of different orders and classes, and it is comparatively easy 
account for the origin of the later more numerous descendants. 
