222 SECRETS OF ANIMAL LIFE 
in function (use and disuse), or in environment and 
nurture generally, ever affect the germ-plasm in the 
reproductive organs in such a specific or representa- 
tive way that the offspring will thereby, though not 
subjected to the nurtural peculiarity in question, 
exhibit the same modification that the parent 
acquired, or even an approximation towards it? 
Modifications are dints of direct extrinsic origin, in 
contrast to variations or mutations which are ex- 
pressions of germinal changefulness; and the pre- 
cise point is whether the acquirer of the modification 
can entail it on his progeny as such or in any repre- 
sentative degree. It is admitted that deep dints 
may have secondary effects on the germ-cells and 
on the unborn offspring; but this is not the question 
at issue. It is also probable that long-continued, 
deeply-saturating peculiarities of nurture may 
produce substances that enter into the germ-cell 
or into the embryonic body (e.g. the mammal in 
its ante-natal life of symbiosis with its mother, or 
the unliberated seed of the flowering plant), but 
there is as yet no convincing evidence that the 
resulting changes grip the constitution permanently. 
It would perhaps facilitate our understanding of 
organic evolution if we found reason to believe that 
at least some advantageous modifications, hammered 
on to the individual, could be transmitted ever so 
little, but the difficulty is to find convincing evi- 
dence. So it has come about, not through any 
preference of darkness to light, but by pressure of 
hard facts, that the majority of naturalists now 
