natural selection. And inasmuch as Weismann believes that such is the case with all the protozoa, as well as with all parthenogenetic organisms, he does not hesitate to accept the necessary conclusion that in these cases natural selection is without any jurisdiction. How, then, does he account for individual variations in the protozoa? And, still more, how does he account for the origin of their innumerable species? He accounts for both these things by the direct action of external conditions of life. In other words, so far as the unicellular organisms are concerned, Weismann is rigidly and unconditionally an advocate of the theory of Lamarck—just as much as in the case of all the multicellular organisms he is rigidly and unconditionally an opponent of that theory. Nevertheless, there is here no inconsistency: on the contrary, it is consistency with the logical requirements of his theory that leads to this sharp partitioning of the unicellular from the multicellular organisms with respect to the causes of their evolution. For, according to his view, the conditions of propagation among the unicellular organisms are such that parent and offspring are one and the same thing; “the child is a part, and usually a half, of its parent.” Therefore, if the parent has been in any way modified by the action of external conditions, it is inevitable that the child should, from the moment of its birth (i.e., fissiparous separation), be similarly modified; and if the modifying influences continue in the same lines for a sufficient length of time, the resulting change of type may become sufficiently pronounced to constitute a new species, genus, &c. But in the case of the multicellular or sexual organisms, the child is not thus merely a severed moiety of its parent; it is the result of the fusion of two highly specialized and extremely minute particles of each of two parents. Therefore, whatever may be thought touching the validity of Weismann’s deduction that in no case can any modification induced by external conditions on these parents be transmitted to their progeny, at least we must recognize the validity of the distinction which he draws between the facility with which such transmission must take place in the unicellular organisms, as compared with the difficulty—or, as he believes, the impossibility—of its doing so in the multicellular.