THE CAUSE OF SPONTANEOUS VAEIATIONS 103 



progression. Consequently progressive evolution can occur 

 only when selection is somewhat stringent, and this is 

 especially the case when a character has been recently and 

 quickly evolved. It follows that, whenever the stringency of 

 selection is relaxed, a character which has undergone 

 evolution tends to regress. 



XY. Regressive variations are absolutely necessary to the 

 existence of complex and variable types. During the develop- 

 ment of every individual many useless variations arise. If 

 these were transmitted and multiplied during subsequent 

 generations, they would render the existence of the species 

 increasingly difficult, and at length impossible. During the 

 evolution of the species many parts become useless. If these 

 were preserved they would render the development of the 

 individual increasingly difficult, and at length impossible. 

 But obviously indiscriminate regression would act as an 

 obstacle to adaptation. Natural Selection, therefore, has 

 evolved bi-parental reproduction, which regulates the tend- 

 ency to reversion in a very beautiful, effective, and dis- 

 criminating manner. It aids Natural Selection to preserve 

 structures and variations when they are useful, and assists 

 regression to eliminate them when they are useless. It is a 

 selective source of regressive variations in offspring ; never a 

 cause of any sort of progressive variations. 



XVI. Natural Selection seizing on progressive variations 

 causes progressive evolution. Bi-parental reproduction 

 seizing on regressive variations causes regressive evolution. 

 The latter is a necessary accompaniment of the former. No 

 doubt Natural Selection may sometimes seize on large and 

 favourable regressive variations, but this can but seldom 

 happen. Natural Selection as commonly understood, there- 

 fore, explains only half the facts of evolution. 



XVII. When Natural Selection is stringent it overcomes 

 the tendency to regression, and progressive evolution ensues. 

 When it is less stringent it merely maintains the structure. 

 When it is still less stringent, and even more when it has 

 been completely abolished, the tendency to reversion comes 

 into play and causes regressive evolution. Natural Selection 

 may be compared to a sculptor who designs and roughly 

 prepares a statue ; reversion, to a colleague, more skilful 

 mechanically, who chisels out its finer lines. 



XVIII. Development is a dim, blurred, and inaccurate 

 recapitulation of the life-history, because (1) many characters 

 have disappeared or are disappearing through reversion, (2) 

 because many characters have been rendered latent, and (3) 



