. It is significant
that the advance of research is multiplying these cases. There is no
reason why we may not assume most of the changes of species we have
yet seen to have occurred in this way. In fact, in some of the lower
branches of the animal world (Radiolaria, Sponges, etc.) there is often
no sharp division of species at all, but a gradual series of living
varieties.

On the other hand we know many instances of very considerable sudden
changes. The cases quoted by Mendelists generally belong to the plant
world, but instances are not unknown in the animal world. A shrimp
(Artemia) was made to undergo considerable modification, by altering the
proportion of salt in the water in which it was kept. Butterflies have
been made to produce young quite different from their normal young by
subjecting them to abnormal temperature, electric currents, and so on;
and, as I said, the most remarkable effects have been produced on eggs
and embryos by altering the chemical and physical conditions. Rats--I
was informed by the engineer in charge of the refrigerating room on
an Australian liner--very quickly became adapted to the freezing
temperature by developing long hair. All that we have seen of the past
changes in the environment of animals makes it probable that these
larger variations often occur. I would conclude, therefore, that
evolution has proceeded continuously (though by no means universally)
through the ages, but there were at times periods of more acute change
with correspondingly larger changes in the animal and plant worlds.

In regard to the issue between the Lamarckians and Weismannists--whether
changes acquired by the parent are inherited by the young--recent
experiments again suggest something of a compromise. Weismann says that
the body of the parent is but the case containing the germ-plasm, so
that all modifications of the living parent body perish with it, and do
not affect the germ, which builds the next generation. Certainly, when
we reflect that the 70,000 ova in the human mother's ovary seem to have
been all formed in the first year of her life, it is difficult to see
how modifications of her muscles or nerves can affect them. Thus we
cannot hope to learn anything, either way, by cutting off the tails of
cows, and experiments of that kind. But it is acknowledged that certain
diseases in the blood, which nourishes the germs, may affect them, and
recent experimenters have found that they can reach and aff