348 LAWS OF VAKIATION. Chap. XXVI 



form. As the supply of organised matter is not unlimited, 

 the principle of compensation sometimes comes into action ; 

 so that, when one part is greatly developed, adjoining parts 

 are apt to be reduced ; but this principle is probably of much 

 less importance than the more general one of the economy 

 of growth. Through mere mechanical pressure hard parts 

 occasionally affect adjoining parts. With plants the position 

 of the flowers on the axis, and of the seeds in the ovary, some- 

 times leads, through a more or less free flow of sap, to changes 

 of structure; but such changes are often due to reversion. 

 Modifications, in whatever manner caused, will be to a certain 

 extent regulated by tbat co-ordinating power, or so-called 

 nisusformativus, which is in fact a remnant of that simple form 

 of reproduction, displayed by many lowly organised beings in 

 their power of fissiparous generation and budding. Finally, 

 the effects of the laws which directly or indirectly govern 

 variability, may be largely regulated by man's selection, and 

 will so far be determined by natural selection that changes 

 advantageous to any race will be favoured, and disadvan- 

 tageous changes will be checked. 



Domestic races descended from the same species, or from 

 two or more allied species, are liable to revert to characters 

 derived from their common progenitor ; and, as they inherit 

 a somewhat similar constitution, they are liable to vary 

 in the same manner. From these two causes analogous 

 varieties often arise. When we reflect on the several fore- 

 going laws, imperfectly as we understand them, and when 

 we bear in mind how much remains to be discovered, we need 

 not be surprised at the intricate and to us unintelligible 

 manner in which our domestic productions have varied, and 

 still go on varying. 



