228 THE EVOLUTION THEORY 



organisms; but the question has not been asked whether this mutual 

 attraction of the unlike really expresses a primary characteristic of 

 organisms, and may not possibly be a secondary acquisition adapted 

 to ensure the occurrence of amphimixis. If we examine the facts we 

 find that even in the lowly Algas, such as Pandorina and Ulothrix, 

 only the migratory cells or swarm-spores of different cell-colonies 

 conjugate with one another, but not those of the same lineage, and 

 this phenomenon may be observed in many unicellular plants and 

 animals. We are justified in concluding from this that a fairly large 

 degree of difference between the conjugating gametes secures the best 

 results, whether this result is to be looked for in a ' rejuvenation ' or in an 

 increased adaptive capacity ; but it is erroneous to regard the stronger 

 attraction between individuals of different descent as a direct outcome 

 of this. To me, at least, it seems to be an adaptive arrangement. 

 The whole of the long and complex phylogenetic history of the sex- 

 cells, the gametes, shows clearly that we have here to deal with 

 a succession of adaptations, and that the degree of attraction which 

 obtains between gametes has gradually been increased and specialized 

 in the course of the phylogeny. I need only briefly recall what we 

 have discussed in a former lecture, that at first the copulating cells 

 were exactly alike in appearance and size, that then one kind of cell 

 became rather larger than the other, and that only gametes which were 

 thus different in size were mutually attractive — the micro-gametes and 

 the macro-gametes, or male and female germ-cells ; we have seen that 

 these differences between the two became more and more accentuated, 

 that the female cell continued to grow larger than the male, and to 

 accumulate more and more nutritive material for the building up of 

 the young organism which arises from its union with the male cell, 

 and that the male cells became smaller, but more numerous, as was 

 essential if their chances of finding the often remote female cell were 

 not to disappear altogether. And besides, there are all the innumer- 

 able adaptations of the egg-cell to the countless special circumstances 

 which obtain in the different groups, and the innumerable varieties 

 in the form of the sperm-cell, with all its delicate and complicated 

 adaptations to the special conditions under which the egg-cell can be 

 reached and fertilized in this or that group of organisms. Of a truth, 

 he is past helping who does not regard with wonder and admiration 

 the adaptations which have been worked out in this connexion 

 in the course of evolution ! But if all these details are adaptations, 

 so is the beginning of the whole process of differentiation ; allogamy, 

 the attraction of conjugating cells of different lineage, is not a primary 

 outcome of individual diversity ; gametes of different descent did not 



Digitized by Microsoft® 



