Manrc# 28, 1884.] 
ovum directly, and the organisms to which they 
give rise will tend to vary in the same manner. 
10. A cell which has varied will continue to 
throw off gemmules, and so cause variations in 
the corresponding parts of the bodies of de- 
scendants, until a favorable variation is seized 
upon by natural selection. 11. The ovarian 
ova will directly inherit the selected variation, 
and will transmit it as an hereditary race charac- 
teristic without the agency of gemmules. 12. 
The occurrence of a variation, but not its pre- 
cise character, is due to the direct action of ex- 
ternal conditions. 
These positions Professor Brooks endeavors 
to establish by a great number of facts, taken 
almost exclusively from Darwin. He first com- 
bats the view that the sexual elements play 
similar parts in reproduction, and the objec- 
tion seems to be well taken; though, when he 
says that it cannot be shown that either sex 
may transmit any characteristic whatever, he 
pushes his objection too far, as is demonstrat- 
ed by a multitude of facts in the breeding of 
domestic animals. 
Having stated the theory, the author devotes 
a large part of his book to the evidence in its 
favor. From the study of hybrids he concludes 
that hybrids and mongrels are highly variable ; 
that the children of hybrids are more variable 
than the hybrids themselves; and that, from 
the evidence of reciprocal crossing in the case 
of hybrids, variation is caused by the influence 
of the male. The evidence from variation is 
then considered, showing that variation is more 
common in sexual than in asexual reproduction 
(in plants at least) ; that changed conditions 
cause variation, not directly, but in subsequent 
generations ; that specific characters are more 
variable than generic; that parts excessively 
developed in males are more variable than 
parts especially developed in females. Profes- 
sor Brooks next takes up the very complex 
subject of secondary sexual characters, and 
shows from various kinds of evidence that the 
male is more variable than the female ; and that 
the male has led the way in evolution, while 
the female has followed. One of the most im- 
portant aspects of the hypothesis, the author 
considers to be the manner in which it removes 
objections to the theory of natural selection, 
by showing that large numbers of animals vary 
similarly and simultaneously, and so give an 
opportunity for natural selection to come into 
play. 
Now, how far can this ingenious and ably 
supported hypothesis be regarded as a perma- 
nently valuable contribution to science? One 
great objection is apparent at the very outset, 
SCIENCE. 
389 
— that the author has not gone to nature for his 
facts, but has taken them almost entirely from 
Darwin’s works, as he candidly says. This 
must necessarily impair the value of his con- 
clusions. The whole work bears the stamp 
of being merely an ingenious attempt to sup- 
plement Darwin’s hypotheses, and re-arrange 
his facts, and might have been written by one 
whose knowledge of biology had been drawn 
almost entirely from Darwin’s books. The 
objection which Mr. Lewes’ made to pangene- 
sis holds equally well against this hypothesis. 
‘¢ The hypothesis is thus seen to be one wholly 
constructed out of suppositions, each and all 
of which may be erroneous, every one of them 
being necessary to the integrity of the scheme.’’ 
Thus, the existence of gemmules is a supposi- 
tion; that cells throw them off when disturbed 
is a supposition ; that the male cell has acquired 
a special power of gathering and storing these 
germs is a supposition. Scarcely a single 
proposition of the hypothesis can be regarded as 
in any way proved. Then, again, some of the 
apparently simple assumptions really involve 
a number of others, equally without evidence. 
Thus, when it is said that the ovarian ova, being 
the direct descendants of the fertilized egg, 
inherit its peculiarities, we have no explanation 
_ offered for what is perhaps as great a mystery 
as the main problem itself. The ovarian ova 
are derived from the fertilized ovum through 
an immense number of intermediate cells, most 
of which become indifferent epithelium. We 
must, then, assume that the gemmules are 
all segregated together, and transmitted un- 
changed from cell to cell till they finally reach 
the ovarian ovum, — surely a very forced sup- 
position. 
The evidence by which Professor Brooks 
endeavors to support his hypothesis is by no 
means convincing: usually all that can be said 
of it is, that it does not contradict the view. 
In spite of his evident candor, the author has 
not always resisted the temptation of straining 
his points to the uttermost limit, often prefer- 
ring a far-fetched and doubtful explanation to 
an obvious one close at hand, as in the cases 
of the zebra and niata hybrids on p. 180. The 
statement that the peculiarities of the niata 
breed of Paraguay cattle are probably due to 
a reversion to the type of Sivatherium will be 
an amusing one to paleontologists. 
Then it is not at all clear, from the evidence 
presented, that this hypothesis will account 
more satisfactorily for the greater development 
of the male in those species in which the sexes 
differ than does Darwin’s theory of sexual 
1 Fortnightly review, new series, vol. iv., p. 508. 
