THE THREE SERIES PARALLEL. 313 



themselves to changed conditions of existence most readily 

 have attained the highest degree of perfection. The 

 further the organic world developed in the course of the 

 earth's history, the greater must the gap between the lower 

 conservative and the higher progressive groups have be- 

 come, as in fact may be seen too in the history of nations 

 In this way also is explained the historical fact, that the 

 most perfect animal and vegetable groups have developed 

 themselves in a comparatively short time to a considerable 

 height, while the lowest or most conservative groups have 

 remained stationary throughout all ages in their original 

 simple stage, or have progressed, but very slowly and 

 gradually. The series of man's progenitors clearly shows 

 this state of things. The sharks of the present day are still 

 very like the primary fish, which are among the most 

 ancient vertebrate progenitors of man, and the lowest 

 amphibians of the present day (the gilled salamanders and 

 salamanders) are very like the amphibians which first de- 

 veloped themselves out of fishes. So, too, the later ances- 

 tors of man, the Monotremata and Marsupials, the most 

 ancient mammals, are at the same time the most imperfect 

 animals of the class which still exist. 



The laws of inheritance and adaptation known to us are 

 completely suflacient to explain this exceedingly important 

 and interesting phenomenon, which may be briefly desig- 

 nated as the parallelism of individual, of palceontological, 

 and of systematic development. No opponent of the Theory 

 of Descent has been able to give an explanation of this ex- 

 tremely wonderful fact, whereas it is perfectly explained, 

 according to the Theory of Descent, by the laws of Inherit- 

 ance and Adaptation, 



