258 
THEORIES OF HEREDITY. 
Nov., 1889. 
Individual variation was believed to be due to the heredi¬ 
tary effect of the direct action of environment. It is known 
that in some cases (e.y. certain plants) variation has been 
caused by the direct action of environment on the germ-cells 
while still contained in the body of the parent. Such a 
change is, of course, blastogenic, and would be transmitted. 
There is less evidence for the operation of such causes in the 
case of animals. The consideration of twins and monstrosi¬ 
ties pointed to the conclusion that individual variation is 
predetermined in the fertilised ovum. If it be asked how 
such differences between ova are produced, Weismann has 
pointed out that there is some evidence that the changes 
which ova and spermatozoa undergo, as a preparation for 
their fusion in fertilisation, must lead to individual differences. 
He, therefore, considers that variation is produced by sexual 
reproduction, and is, in fact, its raison d'etre. The meaning 
of this form of reproduction is to supply variations upon 
which natural selection can operate. 
The apparent effects of increased use are more probably 
due to the operation of natural selection upon a part which 
is, ex hypothesis of especial importance, combined with the 
admitted increase which follows increased use during the 
life of the individual. The apparent effects of disuse are 
more probably due to the cessation of natural selection, 
which can no longer maintain the efficiency of a useless part. 
All functional parts of an organism are kept up to a high 
standard by the operation of natural selection; withdraw 
selection and degeneration will at once begin. It is very 
interesting to find that both Gal ton and Weismann inde¬ 
pendently arrived at the conclusion that this offered a better 
explanation of the gradual dwindling of useless parts, than 
that afforded by the supposition that the admitted dwindling 
which follows from disuse during an individual life is trans¬ 
mitted. 
Finally, the phenomena of instinct seem capable of 
explanation by the operation of natural selection upon blasto¬ 
genic variations of the nervous system, rather than by the 
supposed transmission of acquired habit. In many cases we 
are compelled to adopt the former theory, and it is open to us 
in ail. 
I have not really attempted any discussion of the trans¬ 
mission of acquired characters. I have only indicated the 
chief lines along which the discussion has been and will be 
directed. 
