t that "No
_Puccinia_ but the _Puccinia rosae_ is found upon rose bushes, and this is
seen nowhere else; _Omygena exigua_ is said to be never seen but on the
hoof of a dead horse; and _Isaria felina_ has only been observed upon the
dung of cats, deposited in humid and obscure situations." He adds, "We can
scarcely believe that the air is full of the germs of distinct species of
fungi, of which one never vegetates until it falls on the hoof of a dead
horse, and another till it falls on cat's dung in a damp and dark place."
This is true, but it does not quite follow that they are necessarily the
same species if, as Dr. Bastian seems to show, thoroughly different and
distinct organic forms[108] can be evolved one from another by modifying
the conditions. This observer has brought forward arguments and facts from
which it would appear that such definite, sudden, and considerable
transformations may take place in the lowest organisms. If such is really
the case, we might expect, _a priori_, to find in the highest organisms a
tendency (much more impeded and rare in its manifestations) to {116}
similarly appreciable and sudden changes, under certain stimuli; but a
tendency to continued stability, under normal and ordinary conditions. The
proposition that species have, under ordinary circumstances, a definite
limit to their variability, is largely supported by facts brought forward
by the zealous industry of Mr. Darwin himself. It is unquestionable that
the degrees of variation which have been arrived at in domestic animals
have been obtained more or less readily in a moderate amount of time, but
that further development in certain desired directions is in some a matter
of extreme difficulty, and in others appears to be all but, if not quite,
an impossibility. It is also unquestionable that the degree of divergence
which has been attained in one domestic species is no criterion of the
amount of divergence which has been attained in another. It is contended on
the other side that we have no evidence of any limits to variation other
than those imposed by physical conditions, such, _e.g._, as those which
determine the greatest degree of speed possible to any animal (of a given
size) moving over the earth's surface; also it is said that the differences
in degree of change shown by different domestic animals depend in great
measure upon the abundance or scarcity of individuals subjected to man's
selection, together with the vary