XXV.] GENERAL EEMAKKS ON DEVELOPMENT OF SPECIES. 335 



promote change of the living species in different ways : 

 by causing self-adaptation to the new conditions of life ; 

 by directly stimulating variation, and by selecting those 

 variations which are best adapted to the new conditions. 

 What I wish now to insist on is, that all this will neces- 

 sarily produce " divergence of character ; " for the new 

 influences will most probably act on only part of the indi- 

 viduals of a species, and among those which do come under 

 the new influences it is probable that different variations 

 will be adapted to different habits of life, and will be formed 

 by hereditary habit and natiiral selection into permanent 

 varieties or new species. Thus the constant tendency of Tendency 

 change will be to produce new varieties ; and it appears locr^eal 

 impossible that any retrograde change can reverse this, changes to 



T . ■, n , •, ■ constantly 



or that it can be undone by any cause except tlieir greater 



extinction. Variation. 



It is to be remembered also that with any particular Correla- 

 variation other variations will probably be correlated ; but ya°riations 

 of the laws of such correlations we know nothing. 



It can scarcely be doubted that the lowest organisms are Lowest 

 the most plastic, and consequently the most capable of "Ji^'^jJ^g^g'^^ 

 assuming oi'ganization of a new type. It is probably for plastic, 

 this reason that allied groups, as already remarked, are 

 generally united by their lowest, and not by their highest 

 members. In more technical language, the least differen- classes are 

 tiated species have the greatest capacitv for further dif- u^nally 



„ .. , ^ ., ^ . . "^ . . , united by 



ferentiation, and are capable oi givmg origin to widely their 

 distinct groups above them. Thus, for instance, the lowest ^°'^'^®* 



° -"^ ' ' members. 



air-breathing Vertebrates — the Batrachians — are allied not 

 with the highest, but with some of the lowest fishes. The 

 lowest fishes, on the development theory, have given origin 

 to the higher fishes on the one side, and to the air-breathing 

 Vertebrates on the other. But we cannot assert that this There may 

 is imiversally the case. The affinities of birds, I believe, r*-^ ''^cep- 

 are rather with the higher than with the lower reptiles ; this. 

 and the same is probably true of the mammalia. 



Any intelligent man who has taken the trouble of fol- ^pparent 



. . mconsis- 



lowing my reasoning so far will very probablv make these tencies of 



