SECT. II.] INTRODUCTION. 7 



relations of cause and effect of which Evolution is the expression, 

 but merely as an empirical means of getting at the outward and 

 visible phenomena which constitute Evolution. On the hypothesis 

 of Common Descent, the forms of living things are succeeding each 

 other, passing across the stage of the earth in a constant proces- 

 sion. To find the laws of the succession it will be best for us to 

 stand as it were aside and to watch the procession as it passes by. 

 No amount of knowledge of individual forms will tell us the laws or 

 even the manner of the succession, nor shall we be much helped by 

 comparison of forms of whose descent we know nothing save by 

 speculation. To study Variation it must be seen at the moment 

 of its beginning. For comparison we require the parent and the 

 varying offspring together. To find out the nature of the progres- 

 sion we require, simultaneously, at least two consecutive terms of 

 the progression. Evidence of this kind can be obtained in no other 

 way than by the study of actual and contemporary cases of Varia- 

 tion. To the solution of this question collateral methods of re- 

 search will not contribute much. 



Since Darwin wrote, several of these collateral methods have 

 been tried, and though a great deal has thus been clone and a vast 

 number of facts have been established, yet the advance towards a 

 knowledge of the steps by which Evolution proceeds has been 

 almost nothing. It will not perhaps be wandering unduly if we 

 consider very shortly the reason of this, for the need for the Study 

 of Variation will thereby be made more plain. 



Before the publication of the Origin of Species the work of 

 naturalists was chiefly devoted to the indiscriminate accumulation 

 of facts. By most the work was done for its own sake in the strict- 

 est sense. In the minds of some there was of course a hope that 

 the gathering of knowledge would at last lead on to something 

 more, but this hope was for the most part formless and vague. 

 With the promulgation of the Doctrine of Descent the whole course 

 of the study was changed. The enthusiasm of naturalists ran 

 altogether into new channels ; a new class of facts was sought and 

 the value of Zoological discovery was judged by a new criterion. 

 The change was thus a change of aim, and consequently a change 

 of method. From a large field of possibilities the choice fell 

 chiefly upon two methods, each having a definite relation to the 

 main problem. The first of these is the Embryological Method, 

 and the second may be spoken of as the Study of Adaptation. 

 The pursuit of these two methods was the direct outcome of 

 Darwin's work, and such great hopes have been set on them that 

 before starting on a new line we shall do well to examine carefully 

 their proper scope and see whither each of them may reasonably 

 be expected to lead. 



It is besides in the examination of these methods and in ob- 

 serving the exact point at which they have failed, that the need 

 for the Studv of Variation will become most evident. 



