IN THE THEOEY OF NATURAL SELECTION. 317 



separate stages. We imagine these stages as existing in the con- 

 tinuous course of ontogeny; for here, as in all departments of 

 nature, we make artificial divisions in order to render possible a 

 general conception, and to gain fixed points in the continuous 

 changes of form which have in reality occurred. Just as we dis- 

 tinguish a sequence of species in the course of phylogeny, although 

 only a gradual transition, not traversed by sharp lines of demar- 

 cation, has taken place, so also we speak of the stages of ontogeny, 

 although we can never point out where any stage ends and another 

 begins. To imagine that each single stage of a part is present 

 in the germ, as a distinct group of gemmules, seems to me to be a 

 childish idea, comparable to the belief that the skull of the young 

 St. Laurence exists at Madrid, while the adult skull is to be found 

 in Rome. 



We are necessarily driven to such conceptions if we assume that 

 the transmission of acquired characters takes place. A theory of 

 preformation alone affords the possibility of an explanation : an 

 epigenetic theory is utterly unable to render any assistance in 

 reaching an interpretation. According to the latter theory, the 

 germ does not contain any preformed gemmules, but it possesses, 

 as a whole, such a chemical and molecular constitution that 

 under certain circumstances, a second stage is produced from 

 it. For example, the two first segmentation spheres may be re- 

 garded as such a second stage ; these again possess such a con- 

 stitution that a certain third stage, and no other, can arise from 

 them, forming the four first segmentation spheres. At each of 

 these stages the spheres produced are peculiar to a distinct species 

 and a distinct individual. From the third stage a fourth arises, 

 and so on, until the embryo is developed, and still later the mature 

 animal which can reproduce itself. No one of the parts of such 

 an animal was originally present as distinct parts in the egg 

 from which it was developed, however minute we may imagine 

 these parts to be. If now an inherited peculiarity shows itself in 

 any organ of the mature animal, this will be the consequence of 

 the preceding developmental stages, and if we were able to inves- 

 tigate the molecular structure of all these stages as far back as 

 / the egg-cell, we should trace back to the latter some minute 

 I difference of molecular constitution which would distinguish it 

 from any other egg-cell of the same species, and was destined 



