216 Animal Life and Intelligence. 



subjects of long-continued, assiduous, and careful selection. 

 One cannot be sure whether use has co-operated or not. 



Sufficient has now, I think, been said to show the 

 difficulty of deciding this question, the need of further 

 observation and discussion, and the necessity for a recep- 

 tive rather than a dogmatic attitude ; and sufficient, also, 

 to indicate my reasons for leaning to the view that use and 

 disuse, long- continued and persistent, may be a factor in 

 organic evolution. 



The Nature of Variations. 



The diversity of the variations which are possible, and 

 which actually occur in animal life, is so great that it is 

 not easy to sum up in a short space the nature of variations. 

 Without attempting anything like an exhaustive classifica- 

 tion, we may divide variations into three classes. 



1. Superficial variations in colour, form, etc., not neces- 

 sarily in any way correlated with 



2. Organic variations in the size, complexity, and 

 efficiency of the organs of the body ; 



8. Reproductive and developmental variations. 



Any of these variations, if sufficient in amount and 

 value to determine the question of elimination or not-elimi- 

 nation, selection or not-selection, may be seized upon by 

 natural selection. 



Our domesticated animals exemplify very fully the 

 superficial variations which, through man's selection, have 

 in many cases been segregated and to some extent stereo- 

 typed. It is unnecessary to do more than allude to the 

 variations in form and coloration of dogs, cattle, fowls, and 

 pigeons. These variations are not necessarily in any way 

 correlated with any deeper organic variations. They are, 

 however, in many cases so correlated. For example, the 

 form of the pouter pigeon is correlated with the increased 

 size of the crop, the length of the beak carries with it a 

 modification of the tongue, the widely expanded tail of the 

 fantail carries with it an increase in the size and number 



