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 sufficient to explain this exceedingly important 
and interesting phenomenon, which may be briefly desig- 
nated as the parallelism of individual, of paleontological, 
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. 
