44 PROFESSOR A. M. MARSHALL. 



The advantage of cross-fertilisation in increasing the vigour of the 

 offspring is well known, and in plants devices of the most varied and 

 even extraordinary kind are adopted to ensure that such cross- 

 fertilisation occurs. The essence of the act of cross-fertilisation, 

 which is already established among Protozoa, consists in combination 

 of the nuclei of two cells, male and female, derived from different 

 individuals. The nature of the process is of such a kind that two 

 individual cells are alone concerned in it; and it may, I think, 

 be reasonably argued that the reason why animals commence their 

 existence as eggs, i.e. as single cells, is because it is in this way only 

 that the advantage of cross-fertilisation can be secured, an advantage 

 admittedly of the greatest importance, and to secure which natural 

 selection would operate powerfully. 



The occurrence of parthenogenesis, either occasionally or normally, 

 in certain groups is not, I think, a serious objection to this view. 

 There are very strong reasons for holding that parthenogenetic de- 

 velopment is a modified form, derived from the sexual method. 

 Moreover, the view advanced above does not require that cross- 

 fertilisation should be essential to individual development, but merely 

 that it should be in the highest degree advantageous to the species, 

 and hence leaves room for the occurrence, exceptionally, of partheno- 

 genetic development. 



If it be objected that this is laying too much stress on sexual 

 reproduction, and on the advantage of cross-fertilisation, then it may 

 be pointed out in reply that sexual reproduction is the characteristic 

 and essential mode of multiplication among Metazoa : that it occurs 

 in all Metazoa, and that when asexual reproduction, as by budding, 

 &c, occurs, this merely alternates with the sexual process which, 

 sooner or later, becomes essential. 



If the fundamental importance of sexual reproduction to the welfare 

 of the species be granted, and if it be further admitted that Metazoa 

 are descended from Protozoa, then we see that there is really a con- 

 straining force of a most powerful nature compelling every animal 

 to commence its life history in the unicellular condition, the only 

 condition in which the advantage of cross-fertilisation can be obtained ; 

 i.e., constraining every animal to begin its development at its earliest 

 ancestral stage, at the very bottom of its genealogical tree. 



