II CHANGES IN THE SEXUAL ELEMENTS 41 



as connected with sexual reproduction, is that any new char- 

 acters may produce such a correlative change in the composi- 

 tion, or even in the form of the sexual products, that sexual 

 crossing between the forms concerned is no longer possible. 1 



Often, for example, species quite nearly related show sur- 

 prising differences in their spermatozoa (Rana temporaria and 

 esculenta). 



I have already, years ago, pointed out how inadmissible it 

 is for this reason to base the argument that species are to be 

 considered as independent and isolated from the first, as 

 groups created in their present condition, on the fact that 

 true species are not fertile when crossed with one another. 

 On the contrary, the impossibility, or at least the difficulty, of 

 fertile union, and therewith the separation into species, can 

 be brought about or favoured by any change indirectly or 

 correlatively produced in the sexual products, e.g. the sperma- 

 tozoa, affecting their morphological character, their motion, 

 and material composition. 



The sperm -nucleus and the egg- nucleus must in their 

 physico-chemical properties form with perfect exactness the 

 complements of one another, if through their union a new 

 organism is to be produced. 2 The form and motion of the 



1 Compare on this the remarks in my publication, Ueber den Bau und die 

 Bewegung der Samenfaden in Verhandlungen der physikalisch-medicinischen 

 Gesellscha/L zu Wurzburg, N. Folge, Bd. vi., and Wurzburg, Stahel, 1874. 



2 I must refer in this connection to a conception to which much importance 

 has been attached as a support of Weismann's theory of heredity. The latest 

 researches on fertilisation having shown that sperm - nucleus and egg -nucleus 

 (=: germinal vesicle) do not dissolve in the process, that it is effected not by 

 liquid but by solid substance, therefore the inheritance of acquired characters has 

 been declared impossible, and fertilisation a morphological, not a physico- 

 chemical, combination. On this view we have before us, to consider more fully 

 what we have previously touched upon, an unchangeable, never wearing out, 

 eternally living organic substance (changeable only by sexual combination or by 

 disease), which cannot even be nourished like other parts of the body, for if it 

 were it would necessarily be affected by the constitution of the body. And 

 whence did it originally obtain its peculiar properties ? How and whence has it 

 come to be ? Before we are obliged to ask ourselves these riddles, and before we 



