Review of Darwin’s Theory on the Origin of Species. 179 
be assumed as such, which have diverged enough to reverse the 
movement, to bring out some sterility in the crosses, The best 
marked human races might offer the most likely case. If mu- 
lattoes are sterile’ or tend to sterility, as some naturalists confi- 
dently assert, they afford Mr. Darwin a case in point. If, 
others think, no such tendency is made out, the required evi- 
dence is wanting. 
A fourth and the most formidable difficulty is that of the pro- 
duction and specialization of organs, , 
It is well said that all organic beings have been formed on 
two great laws; Unity of type, and Adaptation to the condi- 
tions of existence.* ‘The special teleologists, such as Paley, oc- 
cupy themselves with the latter only; they refer particular facts 
to special design, but leave an overwhelming array of the widest 
facts inexplicable. The morphologists build on unity of type, 
or that fundamental agreement in the structure of each great 
class of beings, which is quite independent of their habits or con- 
ditions of life; which requires each individual ‘to go through 
a certain formality,” and to accept, at least for a time, certain or- 
gans, whether they are of any use to him or not. Philosophical 
minds form various conceptions for harmonizing the two views 
theoretically. Mr. Darwin harmonizes and explains them natu- 
ly. Adaptation to the conditions of existence is the result of 
atural Selection; Unity of type, of unity of descent. Accord- 
Ype, 
for their specialization, and every adaptation of organ to func- 
y scape natural agencies. 
Whenever he attempts this he reminds us of Lamarck, and shows 
Us how little light the science of a century devoted to structural 
: “re purely natural explanations fail. The organs being given, 
atural selection may account for some improvement; if given 
had act until naturalists have explained how the tadpole is 
us into 
$10 why it isso, the philos ficient and even 
, the philosophy of efficient cause, é 
re whole argument from aeaiifis would stand, upon the admis- 
“a Such a theory of derivation, precisely where they stand 
Outit. At least there is, or need be, no ground of differ- 
.* Owen adds a thi : 4 eee cS be ihe tahis 
kingdom ig 5; rd, viz:—Vegetative Repetition; but this, in the veget | 
is simply Unity of Tym 
