IQO AMPHIMIXIS OR ESSENTIAL MEANING OF [XI I. 



arises in the same way in Infusoria and Metazoa bears a similar 

 significance in both, we may then proceed to the conclusion set 

 forth above that conjugation and fertilization are both essentially 

 concerned with the mingling of the hereditary tendencies of two 

 individuals. 



At the time when I developed this view, which sought the 

 ultimate meaning and true cause of the existence of sexual 

 reproduction in the continual supply of fresh combinations of 

 hereditary tendencies, I contrasted the Metazoa and Metaphyta 

 on the one hand with the Protozoa and Protophyta on the 

 other, and maintained that the chief sources of variability in the 

 former, the multicellular beings, viz. the external influences 

 (including the effects of use and disuse) which alter the body, can 

 have no influence on the processes of selection which alter the 

 species, because their effects are somatogenic and as such cannot 

 be inherited. Only those predispositions can be inherited which 

 are contained in the germ-plasm, but these are either entirely un- 

 influenced by external agencies, or, if altered at all, only very 

 rarely in the same direction as that taken by the somatogenic 

 changes which follow the same cause. Although I naturally 

 did not assume that the germ-plasm itself was entirely un- 

 changed by external influences, the extraordinary persistence 

 of heredity taught me that the change was small and could only 

 take place by imperceptibly small steps. Such causes might 

 well have been the source of the gradual uniform changes in all 

 individuals of a species, if the latter were subjected to the same 

 modifying influences during long series of generations, but not 

 the source of the countless individual differences, ever-varying 

 in direction. This protean individual variability is the indispen- 

 sable preliminary to all processes of selection, and the unceasing 

 mingling of individual hereditary tendencies, which is brought 

 about by sexual reproduction, was in my opinion the source of 

 this variability. I am now, if possible, more firmly convinced 

 than ever of the soundness of this view, and I wish to extend 

 it in one direction. 



At the time I have been speaking of, I looked upon unicellu- 

 lar beings as organisms in which external influences could 

 directly call forth hereditary changes ; for in them reproduction 

 involved the fission of the cell so that changes undergone by 

 the latter must be transmissible to either half. As an example, 



