sect, in.] INTRODUCTION. 15 



All we as yet know is the hist term of the Series. By the 

 postulate of Common Descent we take it that the hist term 

 differed widely from the last, which nevertheless is its lineal 

 descendant: how then was the transition from the first term to 

 the last term effected? If the whole series were before us, should 

 we find that this transition had been brought about by very 

 minute and insensible differences between successive terms in the 

 Series, or should we find distinct and palpable gaps in the Series 

 In proportion as the transition from term to term is minimal and 

 imperceptible we may speak of the Series as being Continuous, 

 while in proportion as there appear in it lacunae, filled by no 

 transitional form, we may describe it us Discontinuous. The 

 several possibilities may be stated somewhat as follows. The 

 Series may be wholly continuous; on the other hand it may be 

 sometimes continuous and sometimes discontinuous; we know how- 

 ever by common knowledge that it is never wholly discontinuous. 

 It may be that through long periods of the Series the differences 

 between each member and its immediate predecessor and successor 

 are impalpable, while at certain moments the series is interrupted 

 by breaches of continuity which divide it into groups, of which the 

 composing members are alike, though the successive groups are 

 unlike. Lastly, discontinuity may occur in the evolution of par- 

 ticular organs or particular instincts, while the changes in other 

 structures and svstems may be effected continuous] v. To decide 

 which of these agrees most nearly with the observed phenomena of 

 Variation is the first question which we hope, by the Study of 

 Variation, to answer. The answer to this question is of vital con- 

 sequence to progress in the Study of Life. 



The preliminary question, then, of the degree of continuity 

 with which the process of Evolution occurs, has never been 

 decided. In the absence of such a decision there has never- 

 theless been a common assumption, either tacit or expressed, that 

 the process is a continuous one. The immense consequence of a 

 knowledge of the truth as to this will appear from a consideration 

 of the gratuitous difficulties which have been introduced by this 

 assumption. Chief among these is the difficulty which has been 

 raised in connexion with the building up of now organs in their 

 initial and imperfect stages, the mode of transformation of organs, 

 and, generally, the Selection and perpetuation of minute variations. 

 Assuming then that variations are minute, we are met by this 

 familiar difficulty. We know that certain devices and mechanisms 

 are useful to their possessors; but from our knowledge of Natural 

 History we are led to think that their usefulness is consequent on 

 the degree of perfection in which they exist, and thai if they were 

 at all imperfect, they would not be useful. Now it is clear that in 

 any continuous process of Evolution such stages of imperfection 

 must occur, and the objection has been raised that Natural 

 Selection cannot protect such imperfect mechanisms so as to lift 



