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 thirty-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, 



buslies, 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 propor- 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 75<> phanerogamic "have their sexes more or less 



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



less than 10S are trees, belong- decidedly separated." 



