EX PERI MEN TA L BIO LOG V 
109 
said to have been as yet demonstrated. But, even then, the 
cardinal principle of evolution, that every structure is either of 
use to its possessor or has been of use to some ancestral form, 
had forced its acceptance upon the minds of all students 
of organic nature. This principle by itself condemns all such 
purely negative criticism as that of “A Field Naturalist” 
as wholly inadequate ; while, whatever may have been the 
rashnesses of many of Darwin’s followers, it also furnished a 
logical basis for a system of deductive biological reasoning. “ A 
Field Naturalist,” dealing mainly, though by no means exclu- 
sively, with the two forms of the common primrose, argues from 
the fact that the short-styled primrose when self-fertilised “ is 
fully and completely fertile,” that the two forms “ are mere 
varieties ” and not a functional variation. He, however, offers 
Trimorphic Heterogony in the Purple Loosestrife. Explained by 
Darwin as an adaptation for reciprocal cross-pollination, and by “ A Field 
Naturalist ” as a mere vaiiaiion. 
{Kindly lint hy Mr. Grant Richards. 
us no explanation of the meaning of these varieties, or of 
the colour, perfume, honey, markings, nectaries, &c., in flowers 
in general. A working hypothesis, when it is found to har- 
monise group after group of facts differing widAy inter se, justifies 
its acceptance as a true theory of Nature’s modus operandi ; and 
this is what has been continuously happening to the theory of 
evolution by natural selection since 1859. One lesson certainly 
has been abundantly forced upon our notice during that period, 
namely that Nature has frequently several widely different 
methods of arriving at the same result. If, as Darwin showed, 
she has various contrivances for favouring cross- pollination, she 
has also, as Professor Henslow has abundantly illustrated, many 
