The changes take place not by the addition of new crystals, but by changing the growth of the original ones." These, however, may be said{115} to be the same species, after all; but recent researches by Dr.H.
Charlton-Bastian seem to show that modifications in the conditions may result in the evolution of forms so diverse as to constitute different organic species.
Mr.Murphy observes[106] that "it is scarcely possible to doubt that the various forms of fungi which are characteristic of particular situations are not really distinct species, but that the same germ will develop into different forms, according to the soil on which it falls;" but it is possible to interpret the facts differently, and it may be that these are the manifestations of really different and distinct species, developed according to the different and distinct circumstances in which each is placed.

Mr.Murphy quotes Dr.Carpenter[107] to the effect 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.