94 THE HISTORY OF CREATION. 
lowest order and of the simplest character, namely, those 
neutral primitive beings which stand midway between 
animals and plants, and on the whole correspond with our 
protista. “These zoophytes,’ he remarks in another pass- 
age, “are the original forms out of which all the organisms 
of the higher classes have arisen by gradual development. 
We are further of opinion that every species, as well as 
every individual, has certain periods of growth, of bloom, 
and of decay, but that the decay of a species is degeneration, 
not dissolution, asin the case of the individual. From this it 
appears to us to follow that it was not the great catastrophies 
of the earth (as is generally supposed) which destroyed the 
animals of the primitive world, but that many survived 
them, and it is more probable that they have disappeared 
from existing nature, because the species to which they 
belonged have completed the circle of their existence, and 
have become changed into other kinds.” 
When Treviranus, in this and other passages, points to 
degeneration as the most important cause of the transforma- 
tion of the animal and vegetable species, he does not under- 
stand by it what is now commonly called degeneration. 
With him “degeneration” is exactly what we now call 
Adaptation or modification, by the action of external 
formative forces. That Treviranus explained this trans- 
transformation of organic species by Adaptation, and its 
preservation by Inheritance, and thus the whole variety of 
organic forms by the inter-action of Adaptation and In- 
heritance, is clear also from several other passages. How 
profoundly he grasped the mutual dependence of all living 
creatures on one another, and in general the wniversal 
connection between cause and effect—that is, the monistic 
