414 SEXUAL RELATIONS OF PLANTS. CHAP. X. 



not have occurred unless cross-fertilisation was already 

 assured, generally by the agency of insects ; but why 

 the production of male and female flowers on distinct 

 plants should have been advantageous to the species, 

 cross-fertilisation having been previously assured, is 

 far from obvious. A plant might indeed produce twice 

 as many seeds as were necessary to keep up its numbers 

 under new or changed conditions of life ; and if it did 

 not vary by bearing fewer flowers, and did vary in the 

 state of its reproductive organs (as often occurs under 

 cultivation), a wasteful expenditure of seeds and pollen 

 would be saved by the flowers becoming diclinous. 



A related point is worth notice. I remarked in my 

 Origin of Species that in Britain a much larger pro- 

 portion of trees and bushes than of herbaceous plants 

 have their sexes separated ; and so it is, according to 

 Asa Gray and Hooker, in North America and New 

 Zealand.* It is, however, doubtful how far this rule 

 holds good generally, and it certainly does not do so 

 in Australia. But I have been assured that the flowers 

 of the prevailing Australian trees, namely, the 

 Myrtaceae, swarm with insects, and if they are dicho- 



* I find in the ' London Cata- ing to thirty-five families. Of 



logue of British Plants,' that there these 108 trees, fifty-two, or 



are thirtj-two indigenous trees very nearly half, have their sexes 



and bushes in Great Britain, more or less separated. Of bushes 



classed under nine families; but there are 149, of which sixty- 



to err on the safe side, I have one have their sexes in the same 



counted only six species of wil- state; whilst of the remaining 



lows. Of the thirty-two trees and 500 herbaceous plants only 121, 



bushes, nineteen, or more than or less than a fourth, have their 



half, have their sexes separated ; sexes separated. Lastly, Prof, 



and this is an enormous proper- Asa Gray informs me that in the 



tion compared with other British United States there are 132 native 



plants. New Zealand abounds trees (belonging to twenty-five 



with diclinous plants and trees ; families) of which ninety-five (be- 



and Dr. Hooker calculates that longing to seventeen families) 



out of about 756 phanerogamic "have their sexes more or less 



plants inhabiting the islands, no separated, for the greater part 



leas than 108 are trees, belong- decidedly teparated." 



