16 Plant-Breeding 



offspring by uniting the features of two individuals into 

 one. There thus arises from every sexual union a number 

 of new or different forms from which nature may select 

 the best, that is, those best fitted to live in the condi- 

 tions in which they chance to be placed. But whilst 

 sex is undoubtedly one of the most potent sources of pres- 

 ent unlikenesses, it is not necessarily an original cause of 

 individual differences, since the two parties to any sexual con- 

 tract must be unlike before they can produce unlike. When 

 once the initial unlikenesses were established, every new 

 sexual union must produce new combinations, so that 

 now, when every new form, from whatever source it 

 appears, comes into existence, there are other intimately 

 related forms with which it may cross. This state of 

 things has existed to a greater or less degree from the 

 moment sex first appeared, so that the organic world is 

 now endlessly varied as the result of a most complex 

 ancestry. 



Physical environment and variation. Every phase and 

 condition of physical circumstances, which are not ab- 

 solutely prohibitive of plant life, have plants which thrive in 

 them. Every soil and climate, every degree of humidity, 

 hills, swamps, and ponds, every place is filled with 

 plants. Even the trunks and branches of trees support 

 other plants, as epiphytes and parasites. That is, plants 

 have adapted themselves to every physical environment ; 

 or, to turn the proposition around, every physical en- 

 vironment produces adaptive changes in plants. There 

 are those, like Weismann and his adherents, who contend, 

 from purely speculative reasons, that these changes do 

 not become hereditary or permanent until they have in- 



