144 Heredity. 



by the splitting of one old one, to parthenogenesis, or re- 

 production from unfertilized oya, without finding any 

 important gap in the series, and we may safely conclude 

 that all these forms of reproduction are fundamentally 

 alike. 



So far as regards the physical side of the problem of 

 heredity, the only essential difference between asexual 

 reproduction and sexual reproduction is the absence 

 of fertilization or union with a male cell in the one case, 

 and its occurrence in the other case. 



It is therefore extremely imjwrtant to compare the two 

 processes, in order to discover Avhether this physical dif- 

 ference is accompanied by any difference in the result. 

 In the one case we have heredity with the male factor 

 omitted, and in the other we have hereditv with a male 

 factor, and if there is any constant difference in the re- 

 sult, we may safely attribute it to this factor. 



In making this comparison we are almost compelled to 

 restrict ourselves to plants, for although asexual repro- 

 duction is not at all unusual in animals, it is restricted, 

 with one exception, to animals which are not domesti- 

 cated or reared by man, and we therefore know too little 

 about the minute details of their life to make use of them 

 for our purpose. The number of plants which have been 

 cultivated and carefully observed and studied by man is 

 very great, and as most of them multiply asexually by bud- 

 ding, as well as by fertilized seeds, w^e here have abundant 

 material for comparative study, and it is well established 

 by hundreds of thousands of observations that the pres- 

 ence or absence of the influence of the male element does 

 have an influence upon the result of the reproductive 

 process, and that this result is exactly what our view 

 of the nature of the process would lead us to expect. 

 Plants produced from fertilized seeds differ from those 



