162 Heredity. 



view that the sexes play simihir parts in heredity than 

 the fact that the offspring of a male hybrid and the fe- 

 male of a pure species is much more variable than the off- 

 spring of a female hybrid by a father of pure blood ? 

 Darwin's pangenesis hypothesis furnishes no explanation 

 of this most remarkable fact, and none of the hypothe- 

 ses of heredity which have been proposed from time to 

 time are sufficiently definite to have any bearing upon a 

 concrete case like this, but our theory that changed con- 

 ditions of life cause a production of gemmules, and that 

 these are stored up in and transmitted by the male ele- 

 ment, fits this case exactly. 



The curious phenomena of reciprocal crosses, again, 

 are just what our theory would lead us to expect, and it 

 also furnishes us with an explanation of the fact that 

 crossing so frequently causes reversion. 



A comparison of sexual with asexual reproduction 

 also gives us a means of analyzing •the influences of the 

 two sexual elements, for asexual reproduction is essen- 

 tially reproduction with the male element left out, and 

 the result of this omission is, as we should expect, the 

 reduction of the tendency to vary to a minimum. At 

 the same time that our theory explains the great rarity 

 of bud variations, it admits of their occasional appear- 

 ance, and it gives an explanation of the singular fact 

 that bud variation is much less rare in plants which 

 have long been cultivated than it is in wild forms. 



The most remarkable of the laws of variation is tlie 

 well-known law that changed conditions do not directly 

 produce variation, but cause subsequent generations to 

 vary. As changed conditions do not in themselves cause 

 hereditary modification, but simply lead to the produc- 

 tion of gemmules, we see why their effect should be 

 manifested in succeeding generations, and we also see 



