324 THE EVOLUTION THEORY 



previously discussed ; all these are only means for bringing about the 

 process of the union of two germ -cells to form a fertilized ' ovum ' 

 which is capable of development. The essential part of this so-called 

 ' sexual reproduction ' does not, however, depend on these differences, 

 neither on the sexual differences of the germ-cells nor on those of the 

 whole organism ; it lies solely in the actual union of the two germ- 

 cells. Let us remember the idea we have already emphasized, that 

 the essential part of the so-called 'sexual reproduction' does not 

 depend on these differences, and let us hold fast to the idea already 

 indicated, that the chromosomes of the nucleus are the real bearers 

 of the hereditary tendencies ; then we see that the mingling, or, better, 

 the union of the hereditary substances of two different individuals, 

 whether single-celled or many-celled, is the result of the process 

 which we have hitherto called fertilization or conjugation, but which 

 we shall henceforward designate by the more general term ' Amphi- 

 mixis,' which means the mingling of substances contributed from two 

 distinct sources. 



Having made ourselves acquainted with the phenomena of 

 amphimixis in animals, plants, and unicellular organisms, we have to 

 face the problem of the significance of this remarkable and com- 

 plicated process. What is it that happens, and what meaning can 

 we attach to it % 



The first thing to be done is to show that the old and long- 

 prevailing conception of fertilization as a life-aivakening process 

 must be entirely abandoned. That a new individual can arise even 

 among highly organized animals, quite independently of fertilization, 

 is proved by the parthenogenetic eggs of insects and crustaceans ; 

 fertilization is not the spark ' which falls into the powder-cask ' and 

 causes the explosion ; it is only an indispensable condition of develop- 

 ment. As we have seen, there are germ-cells which are not sexually 

 differentiated, such as the spores of the lower plants, which are all 

 capable of development without amphimixis ; and parthenogenetic ova 

 prove that even differentiated female germ-cells, that is, germ-cells 

 originally adapted for amphimixis, may in certain circumstances 

 develop without it; amphimixis is thus not the fundamental cause 

 of development, but is only, for many germ-cells, one of the con- 

 ditions which must be fulfilled before development can set in. It 

 is a condition which, under certain circumstances, may be dis- 

 pensed with. 



If, then, the multiplication of individuals by single-celled germs 

 and take place independently of amphimixis, we may conclude that 

 the establishment of amphimixis has nothing to do with the capacity 



