SUMMAEY OF CHAPS. VII TO IX 189- 



haps by germinal selection. These variations are 

 generally small and continuous, but occasionally 

 they take sudden leaps, and then appear to remain 

 fixed. To what this is due we do not know in every 

 case, but the difficulty is not an important one. 

 Variations are preserved by selection or by isola- 

 tion, and are accumulated either by selection or by 

 definite variation. 



It is natural selection, working with other forms- 

 of isolation, which has brought about the main pro- 

 gress of life. It gives a directive or determinate 

 impulse to living organisms, and by its means life 

 has advanced from the lowest Protozoa up to man. 

 But, together with the progressive, or determinate 

 evolution, a large amount of indeterminate or in- 

 definite evolution has been going on, and this has 

 been the result of isolation working alone. We may 

 liken the progress of organic evolution to the march 

 of an army, which is continually throwing off 

 numerous scouting parties., who penetrate into every 

 nook and cranny and leave nothing unexplored. 

 The few which find roads lead off part of the army 

 after them ; while the majority who fail to do so 

 perish on their tracks and are heard of no more. 

 Natural selection preserves and intensifies adapta- 

 tions, or utilitarian characters only; isolation pre- 

 serves both utilitarian and non-utilitarian characters. 

 Progress is due to the former, variety to the latter. 



Xow we rarely see a useful character which is not 

 shared by many species, and consequently is of 

 generic or even of higher value ; while a very large 

 number of specific characters are non-utilitarian. 



