74 'WHAT IS A SPECIES?' 



freshening of the water in which they were kept, until 

 they took on the characters of the genus Branchipus. 

 On this subject Darwin wrote to Karl Semper, February 6, 

 1 88 1 : 'When I read imperfectly some years ago the 

 original paper I could not avoid thinking that some special 

 explanation would hereafter be found for so curious 

 a case. I speculated whether a species, very liable to 

 repeated and great changes of conditions, might not 

 acquire a fluctuating condition ready to be adapted to 

 either conditions.' * 



I venture to express the prediction that this class of 

 cases, already very numerous, will hereafter be im- 

 mensely enlarged, and will become especially important 

 in the vegetable kingdom. 2 Although Hooker at one 

 time took the opposite side, and thought that plants 

 were never ' changed materially by external conditions . . . 

 except in such a coarse way as stunting or enlarging,' 3 



1 More Letters, vol. i, p. 391, Letter 303. 



" See Stimulus and Mechanism as Factors in Organization, by J. Bretland 

 Farmer, F.R.S. (the New Phytologist, vol. ii, Nos. 9 and 10, November and 

 December, 1903). Professor Farmer speaks of the probable prevalence 

 in the plant-world of ' a constant specific mechanism that is able to be 

 actuated in different ways by different kinds of stimuli '. Although for 

 the purpose of his paper Professor Farmer is concerned with the train of 

 physico-chemical sequences which is set going, utility or no utility, when- 

 ever the mechanism of an individual is stimulated, he fully admits that the 

 mechanism itself has come to be a character of the species by the operation 

 of Natural Selection. ' Naturally/ he says, ' only those species whose inner 

 character expressed itself in making these " suitable " adjustments to the 

 environment were able to survive.' 



Toward the close of his paper Prof. Farmer seems to bring the con- 

 siderations that have regard to the species into somewhat unnecessary 

 conflict with those that have regard to the individual. Thus he says that 

 'current literature still teems with teleological explanations that really 

 explain nothing, but rather bar the way of scientific inquiry '. 



A properly loaded, well-constructed modern gun goes off, for disad- 

 vantage no less than for advantage, when its trigger is pulled ; but the 

 very existence of the gun depends upon a long succession of past stages, 

 each of which was more advantageous than its predecessor. The recogni- 

 tion of this history does not bar the way of inquiry, but rather stimulates 

 and suggests a searching and intelligent study of the latest mechanism 

 with all its intricacy. 



8 See the letter from Hooker to Darwin, March 17, 1862, in More 

 Letters, vol. i, p. 197, footnote 2. 



