45 
specific differences have been occasioned by such causes as 
those alleged by Mr. Darwin. This, however, is very far from 
satisfying the hypothesis, which is, not that some races have 
thus originated, but that all have. Mr. Darwin believes 
“ that animals have descended from at most only four or five 
progenitors, and plants from an equal or lesser number.” * 
Analogy, indeed, would lead him one step farther,” namely, 
to the belief that “ all animals and plants have descended from 
some one prototype.” But this he regards as “ immaterial 
whether or not it be accepted, ”t inasmuch as no distinct 
evidence can here be alleged. Confining ourselves, then, to 
the hypothesis proper, the descent of all living beings from 
some eight or ten progenitors, we proceed to our second 
inquiry. 
2. Its adequacy. Are the causes alleged sufficient to ac- 
count for all the specific differences known to exist ? Here 
it is not pretended that more than an approximate answer can 
be given. It is not always possible to give even a probable 
account of how certain differences might thus have been occa- 
sioned. Ail that Mr. Darwin asserts is that his hypothesis 
can be shown to be adequate in so many, even of the appa- 
rently most difficult cases, that there is no valid reason on 
this ground for rejecting it, but rather much reason for 
regarding it as probably true. To estimate the validity of 
this position, it will be necessary to examine somewhat more 
in detail the extent and power of the two great elements in 
the hypothesis above defined — inherited variation and natural 
selection. The necessarily limited space of the present paper 
will render it, however, imperative in doing this to confine 
ourselves to illustrations of the hind of differences capable of 
being thus produced, instead of fully discussing any one or 
more crucial cases. 
That differences in size, in colour, in detailed form, and the 
relative development of different parts, occur in species, and 
are liable to be inherited, no one probably will deny. I restrict 
myself therefore to variations going beyond these. 
And firstly, be it noticed such variations include many 
striking structural changes. Thus we have such cases on 
record as of a woman being born with two or three toes of 
the right foot completely joined together with skin, partially 
webbed in fact ; her children being free from the peculiarity, 
its reappearance however in some of her grandchildren, now 
in the foot, and now in the hand, but always on the right 
side; its perpetuation to her great-grandchildren in like 
* P. 570. 
f P. 571. 
