THE PROBLEM OF SPECIES IN VIEW OF THE ORIGIN OF SOME NEW FORMS IN MICE

Summary. 1. The irradiation of the testicles in mice, with all kinds of doses till that giving complete sterilisation, did not produce in the majority of cases (about 3000 descendants) any hereditary effect. 2. Two mutations only (on 35 irradiated breeders) were obtained: first, a waltzing mouse, breeding true as mendelian recessive, and second, a short‐tailed mouse, so far only known in the hybrid state, but mutating continuously and giving different new forms of tail (anoure, filiforme tail, kinky brachyure, helicoid type, interruption of the skeleton, etc.). 3. These two mutations—“waltzing” and “short tail”—had already been observed in this locality by previous investigators, and besides, one male of our own stock presented a small tail abnormality, which passed unperceived before the irradiation and which proved afterwards to be hereditary. The mutations obtained must not, therefore, be considered as new ones produced by the rays. It was concluded that the rays only revealed a pre‐existing latent state which otherwise would have remained undetected. 4. New evidence supporting the last assumption is the non‐appearance of waltzing or of any tail mutation in a new stock of more than three thousand control mice—descendants from a few normal parents and reared by inbreeding. 5. In the light of these results, the living hereditary material, as represented by different species, may be considered to be quite stable and incapable of being changed in its genetic structure, by any kind of external agency. This accounts for the existence of very ancient species on the earth, for the extreme rarety of the natural mutants, and for the negative results of a great number of attempts to change hereditary behaviour of animals and plants by artificial means. 6. The variability of species and the recorded cases of positive results in attempts to produce a new form by means of different artificial procedures, may be accounted for by the assumption that there exists in different species a certain number of scattered single individuals—potential mutants—whose hereditary material conceals different degrees of instability. 7. This instability probably remains very often unperceived; sometimes it may manifest itself, as well in laboratory conditions as in the state of nature, through spontaneous mutations—source of variation. But the rate of these mutations is in general a very low one, even in Drosophila. This rate may be greatly increased, and the concealed mutability of some apparently normal individuals brought to light, by means of different external agents, especially by irradiating the gonads of the animals used for breeding. 8. The utilisation of penetrating rays seems to be a good method, as they appear to have an elective action on the potential mutants, as was shown by our own findings and by all the recent successful experiments on Drosophila. Yet the rays can hardly be looked upon as a real cause of mutations, and, therefore, the term of “producing” mutations were better abandoned as a misleading one. 9. Hybridisation, natural selection, etc., cannot create new forms. An independent act of nature, creating at least a predisposition to change, precedes all other agencies, though it may not always be at once evident. 10. The rôle of natural selection, as a survival in competitive conditions, is to fix a few of the new forms which are better adapted to a given environment, and to eliminate the majority of them. Hence natural selection cannot be considered as an agent favouring the variability of species on the earth, it is rather an agent limiting this variability. 11. Artificial selection made for purposes of domestication and under laboratory conditions tends, on the contrary, to conserve those mutations appearing spontaneously, or revealed by artificial means, which would perish under nature conditions. 12. This hypothesis of stable species with single changeable individuals amongst them, which are the source of new forms on the earth, would settle numerous experimental controversies. Moreover, it is in full accord with the established impossibility of the inheritance of somatic fluctuations resulting from the adaptation of the individuals to environmental conditions. 13. The present hypothesis conceives the building of organic life resulting from the process of evolution as based on three pillars: (1) stability of existing species as the expression of the conservative principle of life, (2) variability of single individuals as the manifestation of the creative power of nature, and (3) natural selection as the casting away of products which appear less fitted for the struggle for existence under given conditions, and thus determining that the species are adapted to the environment. The struggle for existence plays a special part in phenotypic perfection of individuals.