18 Heredity. 



9. In the higher animals, where the sexes have long 

 been separated the male is more variable than the 

 female. 



10. The result of crossing is not the same when crosses 

 are made reciprocally. 



11. The sex of the parent-species affects the degree of 

 variability of hybrids; and when a hybrid is used as the 

 father, and either one of the pure parent-species, or a 

 third species, as the mother, the offspring are more va- 

 riable than when the same hybrid is used as the mother 

 and either pure parent-species or the same third species 

 as the father. 



There may perhaps be other requisites which should 

 be included in this list, but I think there can be no doubt 

 that a theory of heredity must recognize and be in har- 

 mony with all which are here given. 



2. A sketch of the history of speculation on the 

 theory of heredity. 



The laborious researches of the students of the science 

 of embryology have yielded a rich harvest of valuable 

 facts, and we now know that the process of cell division 

 by which an unspecialized unicellular egg becomes con- 

 verted into a many-celled, highly-specialized organism 

 bears the closest resemblance to the process of growth or 

 of ordinary cell-multiplication. 



We know that all the various forms of reproduction, 

 cell-multiplication, fission, gemmation, conjugation, 

 sexual reproduction, and parthenogenesis, are inter-re- 

 lated in such a way that we must believe that they are 

 different manifestations of the same power, and that they 

 have been evolved one from the other. 



We know that direct development, metamorphosis, and 

 alternation of generations are not separated from each 

 other by any hard and fast line? and we know too that 



