124 GEOLOGICAL BIOLOGY. 



elusive mark or seal on each living type." And in the dis- 

 tribution of fossil as well as living species was seen evidence 

 of " relationship of descent" and of "the derivation from an 

 original protoplast." But descent was supposed by him and 

 his school to be without modification; it was the transmission 

 without change of the ancestral characters to their offspring. 

 Whatever modification might appear was considered an irreg- 

 ularity of individual growth, the cause of which was looked 

 for in idiosyncrasies of the individual or in accidents of en- 

 vironment. Forbes was not ignorant of the paleontologic 

 succession of species. Ancestry determined the specific 

 characters, but it was supposed to determine their likeness,, 

 and not their differences. All the evolving of new forms was 

 traced to antecessory causes and conditions, but the immedi- 

 ate ancestors, it was believed, were capable of transmitting 

 only the characters which they received from their ancestors. 

 There is nothing wrong with "geographical distribution," or 

 "specific centres," or "specific characters," as used by the 

 older naturalists; the new light has come into the interpreta- 

 tion of descent and the nature of species. 



The Meaning of Evolution by Descent. It is important to dis- 

 tinguish between the names of things and their explanation. 

 The term evolution by descent is in this respect faulty, for it 

 means both more and less than is intended. More, in that 

 the most important factor brought forward in explanation of 

 evolution to-day, that of natural selection, is among the 

 extrinsic rather than the intrinsic forces, when the conditions 

 of environment are strictly discriminated; while descent, or 

 ancestry, can be applied only to those forces or conditions 

 which are intrinsic. It expresses less than is intended in that 

 it is not meant that descent alone determines the steps of 

 evolution. 



Distinction between Evolution and Development. Huxley's 

 definition, "evolution, or development, is, in fact, at present 

 employed in biology as a general name for the history of the 

 steps by which any living being has acquired the morpho- 

 logical and the physiological characters which distinguish it," 

 is defective in that it includes a definition of both evolution 

 and development. Development of the individual organism, 



