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, as in the case of the individual. From this it 

 appears to us to follow that it was not the great catastrophes 

 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 universal 

 connection between cause and effect — that is, the monistic 



