On the other hand, what always has been and still remains hypothetical is the explanation of the descent of all plants and animals by natural selection alone. We have already spoken of the _mutations_ of _de Vries_, and the theory of the _mneme_ elaborated by _Semon_, and need not repeat them here. Thanks to the idea of _Hering_, worked out by _Semon_, the facts are now explained in a satisfactory manner. Engraphia, produced in the organisms by the irritating agents of the external world, prepares and builds up little by little their increasing complications, while selection, by continually eliminating the unfit, directs the elaborating work of the mneme and adapts it to the surrounding local circumstances. _De Vries_ has objected that the variations produced by artificial and natural selections are mutable, while sudden mutations have a much more stable character. But we have just seen that these mutations themselves are evidently only the delayed ecphoria of a long ancestral engraphia accumulated. On the other hand, the variations obtained by selection are themselves only due to more rapid ecphorias, derived from repeated conjugations in a certain direction. _Plate_ and others have shown that they may become more and more fixed, if they are well adapted, and thus become more tenacious. There is, therefore, no contradiction between the fundamental facts, and all is simply and naturally explained by the combination of hereditary mnemic engraphia with selection. Recent study on the transformations of living beings have shown that they do not take place in a regularly progressive manner, as _Darwin_ at first believed, but that periods of relatively rapid transformation alternate with periods of relative arrest, both in a general way and for each particular species. We see certain species remaining almost stationary for an immense time and tending rather to disappear, while others vary enormously, showing actual transformation. The transplantation of one species to a new environment, for instance to a new continent, provokes, as has been proved, a relatively rapid transformation. It is evident that mnemic engraphia transforms organisms the more rapidly as it changes in nature itself, which is the case in the migrations we have just mentioned, and which also changes the factors of selection. Other facts show clearly that the fauna and flora of the present world find themselves in a period of recoil with regard to their modification. In the tertiary period the fauna and flora of the world were richer than to-day; many more older species have disappeared than new ones have arisen. This fundamental fact seems due to the extremely slow cooling of the earth, and appears to be indicated by the powerful growth in tropical climates, the fauna and flora of which resemble those of the tertiary period, and, on the other hand by the relative poverty and slowness of growth in cold countries.