THEORY OF NATURAL SELECTION. 195 



infer from the mere fact of the parts in question differing 

 or varying greatly on the same plant, that such modifica- 

 tions were of extremely small importance to the plants 

 themselves, of whatever importance they may generally 

 be to us for our classifications. The acquisition of a use- 

 less part can hardly be said to raise an organism in the 

 natural scale ; and in the case of the imperfect, closed 

 flowers, above described, if any new principle has to be 

 invoked, it must be one of retrogression rather than of 

 progression; and so it must be with many parasitic and 

 degraded animals. We are ignorant of the exciting cause 

 of the above specified modifications ; but if the unknown 

 cause were to act almost uniformly for a length of time, 

 we may infer that the result would be almost uniform ; and 

 in this case all the individuals of the species would be 

 modified in the same manner. 



From the fact of the above characters being unimpor- 

 tant for the welfare of the species, any slight variations 

 which occurred in them would not have been accumulated 

 and augmented through natural selection. A structure 

 which has been developed through long-continued selec- 

 tion, when it ceases to be of service to a species, generally 

 becomes variable, as we see with rudimentary organs ; for 

 it will no longer be regulated by this same power of selec- 

 tion. But when, from the nature of the organism and of 

 the conditions, modifications have been induced which are 

 unimportant for the welfare of the species, they may be, 

 and apparently often have been, transmitted in nearly the 

 same state to numerous, otherwise modified, descendants. 

 It cannot have been of much importance to the greater 

 number of mammals, birds, or reptiles, whether they were 

 clothed with hair, feathers, or scales ; yet hair has been trans- 

 mitted to almost all mammals, feathers to all birds, and 

 scales to all true reptiles. A structure, whatever it may 

 be, which is common to many allied forms, is ranked by 

 us as of high systematic importance, and consequently is 

 often assumed to be of high vital importance to the species. 

 Thus, as I am inclined to believe, morphological differences, 

 which we consider as important — such as the arrangement 

 of the leaves, the divisions of the flower or of the ovarium, 

 the position of the ovules, etc., first appeared in many cases 

 as fluctuating variations, which sooner or later became con- 

 stant through the nature of the organism and of the sur- 

 rounding conditions, as well as through the intercrossing of 



