SEXUAL REPRODUCTION. 153 



between unicellular 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 

 fertilisation may be arranged in the following series : — 



(a) The formation of plasmodia. 



(If) Multiple conjugation. 



(<r) Conjugation of two similar cells. 



(^) Union of incipiently dimorphic cells. 



(e) Fertilisation 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 conjugation, but has since withdrawn this view mainly on the 

 ground that the nuclei have not been shown to coalesce. Now 

 there seems no result of studies on fertilisation more certain 

 than that the union of nuclei is an essential fact, but in plasmo- 

 dium-formation, such intimate association of nuclei cannot 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 behaviour 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 con- 

 ception 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 practi- 

 cable than it was a few years ago. 



§ 7. Hybridisation in Ani)iials. — 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 

 lo 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, 

 N^ have been successfully crossed. In nature, however, we know very little 

 X of the occurrence of any such hybridisation. It seems to occur in some 

 \ishes ; 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 demarcation of a species is the vague line which marks the 



