TUE EVOLUTION-HYPOTHESIS. 433 



the probability that evolution will presently be recognized as 

 the law of the phenomena we are considering? Each further 

 advance of knowledge confirms the belief in the unity of 

 Nature; and the discovery that evolution has gone on, or is 

 going on, in so many departments of Nature, becomes a 

 reason for believing that there is no department of Nature in 

 which it does not go on. 



118. The hypotheses of Special Creation and Evolution, 

 are no less contrasted in respect of their legitimacy as hy 

 potheses. While, as we have seen, the one belongs to that 

 order of symbolic conceptions which are proved to be illusive 

 by the impossibility of realizing them in thought; the other 

 is one of those symbolic conceptions which are more or less 

 fully realizable in thought. The production of all organic 

 forms by the accumulation of modifications and of diver 

 gences by the continual addition of differences to differences, 

 is mentally representable in outline, if not in detail. Various 

 orders of our experiences enable us to conceive the process. 

 Let us look at one of the simplest. 



There is no apparent similarity between a straight line 

 and a circle. The one is a curve; the other is defined as 

 without curvature. The one encloses a space; the other 

 will not enclose a space though produced for ever. The one 

 is finite ; the other may be infinite. Yet, opposite as the two 

 are in their characters, they may be connected together by a 

 series of lines no one of which differs from the adjacent ones 

 in any appreciable degree. Thus, if a cone be cut by a plane 

 at right angles to its axis we get a circle. If, instead of 

 being perfectly at right angles, the plane subtends with the 

 axis an angle of 89 59 , we have an ellipse which no human 

 eye, even when aided by an accurate pair of compasses, can 

 distinguish from a circle. Decreasing the angle minute by 

 minute, this closed curve becomes perceptibly eccentric, then 

 manifestly so, and by and by acquires so immensely elongated 

 a form so as to bear no recognizable resemblance to a circle. 



