Genetics and Evolution 
125 
only remotely concerned, so has Weismann over-emphasised the power of 
selection to the exclusion of everything else. And de Vries, who believed he 
had witnessed a striking instance of mutation, spontaneous origin of species, 
has come to believe mutation to be the sole important cause of evolution.” 
But the modern geneticist, for the most part, takes little interest in the 
problems of evolution, perhaps by way of reaction from the excessive specu¬ 
lation and fine-drawn theorising of the post-Darwinian period at the end of 
last century. “No one, since Darwin,” our authors consider, “has set forth a 
comprehensive theory of evolution worth the name.” 
“Every theory of evolution must account for variation, must give a 
plausible explanation of the causes of that variation which may be instrumental 
in species formation, and in the second place it must account for specific 
stability.... Lamarck thought that the stability of a species is obtained.... as 
soon as the species has come into a new state of equilibrium with its sur¬ 
roundings. .. .Weismann thought of the final stability and purity of a species 
as the result of a long continued natural selection. De Vries holds that the 
unknown causes for the abrupt variation which produces a new species imply 
a new stability.” 
In regard to heredity the authors rightly lay stress on the great advance 
made by Roux when he distinguished determination factors from realisation 
factors. We now believe, as has been said, that “ every character of an organism 
is both inherited and acquired.” “Genetics,” according to the Hagedoorns, 
“is essentially a branch of biomechanics, concerning itself with a study of 
those factors in the development of an organism which are inherited.” They 
believe that “numerous things are transmitted from parent to offspring, each 
of which, by its presence in the cells, tends to influence one or more definite 
steps or processes in development, whenever these steps are taken or these 
processes undergone.... no latency or semi-latency need be ascribed to these 
inherited things {genes ), which in certain individuals are not factors in develop¬ 
ment and which nevertheless are transmitted by them to some of their 
children 1 .” All inherited “factors” are genes but all the genes present in the 
germ are not necessarily factors in its development. The presence or absence 
of a certain gene may determine a definite difference in the final qualities, 
but it is inadmissible to speak of such a gene as the “determiner” of that 
quality, for all the other genes contributing to the developmental process 
which results in the character in question could each, and all in their turn, be 
called its determiner. The characters of an organism are not so many separate 
things, they are all the result of a great many factors, some inherited (genes), 
some furnished by the environment. We have every reason to believe now that 
every gene is present in the zygote in the same state, that every zygote is a 
fresh beginning, that in so far as an individual’s character can be said to be 
determined in its germ, it is given in the combination of genes present, not in 
peculiar states of them. 
“A vitalistic view of the nature of genes certainly fits the facts, but whereas 
1 Thus for instance there is a gene in rats, which when present in coloured animals 
makes otherwise black animals “agouti,” but in albino rats the same gene, though 
it has no influence upon colour, is nevertheless transmitted in precisely the same way 
as in families of animals in whose development it plays an active part. 
