SEXUAL REPRODUCTION. l ,, 



trast to one another and in opposition to the general cells of the 

 body. A consideration of the phenomena in loose protist colonies 

 like Volvox or Ampullina, which suggest the bridge between uni- 

 cellular and multicellular organisms, shows how gradually this latter 

 contrast also may have been brought about. 



To sum up, the steps in the development of the process of fertili- 

 zation may be arranged in the following series: — 



(a) The formation of plasmodia. 



(b) Multiple conjugation. 



(V) Conjugation of two similar cells. 



(d) Union of incipiently dimorphic cells. 



(e) Fertilization by differentiated sex-elements. 



One difficulty must in fairness be allowed in connection with the 

 hypothesis of deriving conjugation from plasmodial union. Some 

 years ago, Sachs was inclined to regard the plasmodium formation of 

 Myxomycetes as a process of multiple conjugation, but has since with- 

 drawn this view mainly on the ground that the nuclei have not been 

 shown to coalesce. Now there seems no result of studies on fertiliza- 

 tion more certain than that the union of nuclei is an essential fact, 

 but in plasmodium-formation, such intimate association of nuclei can 

 not be asserted. The difficulty of making this a starting-point is thus 

 at first sight considerable. 



Yet it must be observed (i) that our knowledge of the nuclei in 

 those lowly forms is still very inadequate; (2) that, according to Gruber, 

 the behavior of the nucleus is sometimes masked by the fact, that 

 instead of existing as a discrete body in the cell, it lies diffusely in the 

 protoplasm; but especially (3) that it is quite consistent with the general 

 evolutionary conception to suppose that the primitive union was of 

 very much less definite character than that subsequently evolved. A 

 reinvestigation of the whole question of plasmodium formation, from 

 this point of view, is however very desirable, especially since the 

 recent progress of microscopic technique has rendered the study of 

 the nucleus in the lowest forms much more practicable than it was a 

 few years ago. 



VII. Hybridization in Animals. — Many of the compound names of 

 animals, such as leopard, point back to a once prevalent belief that animals 

 of very different kinds might unite sexually and have fertile offspring. Only 

 to a very limited extent is such a notion justified. Every one is aware that by 

 direct human control animals like horse and ass, dog and wolf, lion and tiger, 

 hare and rabbit, canary and finch, pheasant and hen, goose and swan, have 

 been successfully crossed. In nature, however, we know very little of the 

 occurrence of any such hybridization. It seems to occur in some fishes ; 

 different species of toad are often seen in sexual union, but the result is 

 unknown ; in higher animals it seems confined to varieties of a species. The 



