THE GENERAL SIGNIFICANCE OF AMPHIMIXIS 211 



Why, then, is there such an intinicate, and in the case of the 

 higher types, such an indissoluble, association between reproduction 

 and amphimixis that ' fertilization ' appears to be a sine qua non of 

 reproduction, and not very long ago seemed to us to be the ' quicken- 

 ing of the ovum,' the ' burning spark ' which causes the powder-barrel 

 to explode ? 



The reason of this is not difficult to discover; it lies in the 

 structure of multicellular animals, and in their differentiation accord- 

 ing to the principle of division of labour, for since only particular 

 cells are capable of reproduction, that is, of giving rise to the whole, 

 it is in these necessarily that the process of amphimixis has to occur 

 if its significance lies in its effects on the succeeding generations. It 

 is true that in the lowest multicellular organisms, such as the species 

 of Volvox, there are, in addition to the sex-cells, other reproductive 

 cells quite similar to the ova, whose development into a new colony 

 takes place without amphimixis, but the higher we ascend in the 

 animal and plant series the rarer are these ' asexual ' germ-cells or 

 ' spores,' and in the highest animal types they are entirely absent and 

 reproduction occurs only by means of the ' sex-cells.' 



I am inclined to look for the cause of this striking phenomenon 

 mainly in the fact that, if amphimixis had to be retained, this was 

 effected with increasingly great difficulty the more highly and com- 

 plexly differentiated the organisms became, and that more complicated 

 adaptations were therefore necessary in order that the union of the 

 two germ-cells might be rendered possible at all. There is first of 

 all the separation into two kinds of sex-cells, whose far-r-eaehing 

 differentiations and precise adaptations to the most minute conditions 

 we have already discussed ; then follow the innumerable adaptations 

 to bring about the meeting of the sex-cells, the arrangements for 

 copulation, and, finally, the instincts which draw the two sexes 

 together, the means of attraction which are employed, whether 

 decorative colours or attractive shapes, stimulating odours or musical 

 notes, in short, all the diverse and intricate arrangements, which seem 

 to be more subtly elaborated the higher the organism stands upon 

 the ladder of life. When we call to mind that sexual differentiations 

 finally go so far that they dominate the whole organism, alike in its 

 external appearance and in its internal nature, its feelings, inclinations, 

 instincts, its will and ability, as well as its structure down to the 

 finest nerve-elements, we can understand that a mode of reproduction 

 which demands such a composite disposition of details, involving 

 a moulding of the whole organism, so to speak, from birth till death, 

 must of necessity remain the only one, and that there was no room 



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