A singularly instructive field for the study of the mutability or stability of species should be afforded by the lower forms of life, in which organization is reduced to a minimum, they being mere masses of protoplasm without even a containing envelope, while their nourishment is of the simplest. It would therefore appear that environment should be all-potent to modify them and produce specific modifications, while the extreme rapidity with which they propagate their kind, and that unisexually, ought to require no vast extent of time to make such transmutations apparent. It is found, however, on the contrary, that nowhere in organic nature does the type remain more rigidly persistent. Professor Macbride, for example, tells us,[327] "The Myxomycetæ may be regarded as the organic group in which the forces of heredity,--whatever these forces may be--are at their maximum: they have responded as little as possible to the influence of their environment." To the same effect speaks Professor Paulesco of Bucharest, of other elementary organisms.[328] What is still more remarkable, these same organisms are extremely sensitive to altered conditions of environment, which have a direct and immediate influence, gravely modifying their morphological and physiological characters, changes in respect of light, minute alterations of temperature, or the introduction of a new chemical substance, even in infinitesimal quantity, frequently causing them to assume forms very different from the specific type, and profoundly modifying their nutritive processes. Here, it was at first thought, when Pasteur revealed their history, is clear evidence of specific transformation. But he presently convinced himself and others that it is not so, for although liable to assume such polymorphic forms according to the conditions in which they find themselves, there is no alteration of specific nature, and if the original circumstances be restored, the original forms reappear--"une élasticité functionelle de la cellule lui permettant de se plier à des conditions variées d'existence sans changer d'être." (Pasteur.) As M. Duclaux adds:[329] "La notion d'espèce ne disparait pas pour cela. La variabilité est un caractère comme un autre, bien que plus difficile à inscrire dans la classification, et une espèce est aussi bien définie par les sensibilités diverses qu'elle manifeste que par la petite liste des mots et de propriétés dans laquelle on croyait pouvoir autrefois enfermer toute son histoire.... La lien de l'espèce c'est la loi qui préside à ces changements, et la variété des formes et des fonctions n'est pas du tout en contradiction avec l'unité de l'espèce." _B._ _Note on Chap. XV. p. 203._ Since the foregoing pages have been in type there has come to hand the New York _Literary Digest_ of January 23, 1904, containing the following article (p. 119). "ARE THE DAYS OF DARWINISM NUMBERED?"