ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
35& 
greater frequency of unfavourable variations to quantitative variations 
among the parts of an organ, and lias committed himself to the sen- 
tence that “ an organ at the height of adaptation cannot vary in the 
direction ‘ better/ for every independent deviation of its parts must 
have the import of 4 worse.’ ” But this seems to Wolff to be sawing 
at the branch which supports the whole Darwinian theory of pro- 
gressive evolution. What applies to the disappearance of an organ 
must also apply to its establishment ; Weismann postulates for both no 
longer a fortuitous crop of variations, but a definite variation-tendency 
towards plus when an adaptive character is being evolved, towards minus 
when it is disappearing. 
But even this concession may, according to Weismann, be made to 
consist with Darwinism ; he tries to effect this by an ingenious combina- 
tion of his Germ-plasm theory with Roux’s theory of the struggle of 
Parts. Of these two theories Wolff then gives a short account. 
The main idea of germinal selection is that the determinants, like 
other parts of the organism, are subject to struggle and intra-selection, 
in the course of which the stronger become stronger (in assimilative 
power, &c.), and the weaker become weaker, so that an internal tendency 
is established which emphasises lines of variation already started and 
sanctioned by the ordinary selection of individuals. 
To this Wolff answers as follows : — (1) The theory leaves the be- 
ginnings of the “ adaptive variation-tendency ” unexplained, thus resem- 
bling Darwinism in general, the fundamental error of which is that it 
works only with continuations, not with beginnings. But this is an old 
story. 
(2) Supposing that a smaller determinant corresponds to a smaller 
organ, does it follow that the determinant is weaker in assimilative 
power ? Smallness and atrophy cannot be identified. If smaller organs 
arise from determinants which have been weakened in their assimilative 
power, then, according to Weismann, this assimilative power must con- 
tinually decrease. But how then can we understand that a small organ 
persists at all, and may in certain cases even increase again ? Moreover, 
by what conceivable process — functioning, trophic stimulus, &c. — can 
the assimilative power of the primarily stronger determinants be further 
increased ? 
(3) Finally, Wolff asks what are the real competitors or combatants 
in the alleged process of germinal selection. Are they the component 
parts of the germ-plasm ? But these stand in a quite different relation 
to one another from that occupied by individual organisms in nature, or 
even by the parts of the body. The factor of “ over-production ” is inap- 
plicable to the germ-plasm, in which the essential parts of the future 
organism are represented only once. 
The author ends by expressing his conviction that the attempt to 
find a “ mechanical ” explanation of adaptive variation has proved a 
failure. That may be ; but we miss in this lecture any indication of 
how the author himself proposes to meet the acknowledged difficulties 
of the evolution-problem. 
Correlated Variation.* — Messrs. C. B. Davenport and C. Bullard 
have studied the variations in the Mullerian glands that are found on. 
* Proc. Amer. Acad., xxxii. (1896) pp. 87-97. 
