AMPHIGONIC HEREDirV' 23 1 



question as far as is necessary in order to render comprehensible 

 the complications in the phenomena of heredity resulting from 

 them. 



Until far into the present century •• sexual reproduction ' was 

 considered to be the essential and primary form of the process ; 

 and although it gradually became more and more evident that 

 several kinds of * asexual reproduction ' may also occur, these 

 nevertheless only take place in the lower forms of animals and 

 plants. As the details of the phenomena of reproduction were 

 for a long time known almost exclusively with regard to the 

 higher animals, and as in them sexual reproduction alone occurs, 

 the special peculiarities of the latter were naturally considered to 

 be necessary and indispensable in the process. Fertilisation 

 was looked upon as an essential part of this process, and it alone 

 was supposed to render life from one generation to another pos- 

 sible at all ; in short, fertilisation was regarded as a ' process of 

 rejuvenescence,' and sexual reproduction was considered to form 

 the foundation from which all forms of reproduction have arisen. 

 The existence of different forms of asexual reproduction was ex- 

 plained as an • after-effect ' of the process of fertilisation or reju- 

 venescence occurring in sexual reproduction. 



This view appeared to receive support from the fact that sex- 

 ual reproduction is of universal occurrence, from the lowest to 

 the highest forms of animals and plants : and also that asexual 

 reproduction never takes place in the higher organic forms, and 

 even in the lower ones it only alternates with the sexual form of 

 the process. 



The present state of our knowledge of the process of fertilisa- 

 tion, however, justifies us in considering these earlier views to be 

 totally erroneous. In no case does fertilisation correspond to a 

 rejuvenescence or renewal of life, nor is its occurrence necessary 

 in order that life may endure : it is merely an arrangement whicJi 

 renders possible tJie i)itermingling of two different hereditary 

 tendencies. We shall deal later on with the question as to why 

 such a mingling has been introduced and so extensively adopted 

 by Nature : at present it is only necessary to prove that this is 

 the case. Fertilisation consists in the union of two hereditary 

 substances, i.e.. of the germ-plasms of two individuals: all the 

 complicated and varied phenomena of differentiation — beginning 

 with that of the two different kinds of reproductive cells, usually 

 known as male and female, up to that of the individuals them- 



