REVIEWS. 
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he objects rather to the extreme arguments of the external-conditionists, if 
we may so style them, than assumes the existence of so illogical a principle 
as a natural tendency. On the whole, we are disposed to believe that Mr. 
Darwin believes that species vary in every generation from the action of 
external conditions, but that he would employ the expression external con- 
dition ” in its widest sense, and would not limit it, as some folk do, to tem- 
perature and atmospheric changes. There can, we think, then, be little 
doubt that Mr. Darwin has demonstrated the following propositions : — 
1st, that in every group of animals the individuals tend constantly to 
diverge by variation from the parent type j 2nd, that these variations are 
in heritable under certain definite laws already expressed 3rd, that by 
selecting and breeding from animals with any particular phj'sical quality, 
that quality may be enhanced through successive generations till it becomes 
a permanent character j and, 4th, that the differences thus produced are 
quite as formidable as those on which naturalists base their distinction of feral 
species. Thus far, and no farther. There come, then, for consideration the 
questions of Mr. Darwin’s opponents, who say, “You have undoubtedly pro- 
duced new animal forms, which seem worthy of being called species, but for 
this fact, that natural species are sterile inter se. Your species are perfectly 
fertile, and if allowed to breed together would produce mongrels, and 
revert to the parent species. Until you explain this anomaly, we decline to 
accept your views.” This is certainly the most difficult point in the whole 
controversy, and this is Mr. Darwin’s reply to it : — 
“ Passing over the fact that the amount of external difference between two 
species is no sure guide to their degree of mutual sterility, so that similar 
differences in the case of varieties would be no sure guide, we know that 
with species the cause lies exclusively in differences in their sexual consti- 
tution. Now the conditions to which domesticated animals and cultivated 
plants have been subjected have had no little tendency towards modifying 
the reproductive system in a manner leading to mutual sterility. But we 
have good grounds for admitting the directly opposite doctrine of Pallas — 
namely, that such conditions generally eliminate this tendency j so that the 
domesticated descendants of species which, in their natural state, would have 
been in some degree sterile, when crossed become perfectly fertile together. 
With plants, so far is cultivation from giving a tendency towards mutual 
sterility, that in several well-authenticated cases already often alluded to, 
certain species have been affected in a very different manner, for they have 
become self-impotent, whilst still retaining the faculty of fertilising and 
being fertilised by distinct species. If the Pallasian doctrine of the dimi- 
nution of sterility through long-continued domestication be admitted — and 
it can hardly be rejected — it becomes in the highest degree improbable that 
similar circumstances should commonly both induce and eliminate the same 
tendency; though in certain cases, with species having a peculiar constitu- 
tion, sterility might occasionally be thus induced.” 
We have already so far transgressed the limits allotted to us, that we have 
barely space to call attention to Mr. Darwin’s theory of Fangenesis. This, 
which is a modification of Reaumur’s and Bonnet’s (see Quatrefage’s Meta- 
morphoses of Man and the Lower Animals : Hardwicke) Panspermy, may be 
shortly expressed as follows : — The tissues of the body in both male and female 
