May 27, 1898.] 



SCIENCE. 



729 



form of the bodies exhibiting them. They 

 are: (a) Growth, strictly speaking Metabo- 

 lism ; (6) Development (the second part of 

 the factor called Eeproduction by Darwin) , 

 and (c) Divergence of Character — properly 

 Evolution — which includes the phenomena 

 of variation. 



II. The second group of factors are all of 

 a secondary nature. They are: (a) the first 

 part of Reproduction, i. e. Generation, 

 which is seen in its simplest form in Mitosis, 

 next in cell-cleavage, in which the process 

 results in producing two more or less equal 

 and similar parts ; and only in organism 

 of some degree of differentiation in struc- 

 ture does it result in true generation through 

 the formation of immature germ-cells, which 

 continue development along hereditary 

 lines of generation ; (6) Inheritance, which 

 cannot take place till inequality between 

 germ and parent is already attained at the 

 time of generation, and the attainment of 

 this inequality cannot be primitive ; (c) 

 Ratio of Increase ; (d) Struggle for life, and 

 (e) Natural Selection, none of which can 

 occur till after generation and inheritance 

 have resulted in the production of antagon- 

 istic individuals. 



The fundamental genetic phenomena of 

 the first group are related to each other, 

 and therefore distinguished, in the same 

 way as the fundamental phenomena of non- 

 living matter are related and distinguished 

 in chemical, molecular and molar groups. 

 Metabolism pertains to changes in the mo- 

 lecular relations of the contents of a living 

 cell ; Development pertains to transmuta- 

 tions of the cellular contents (as cells, tis- 

 sues andorgans) of a living unit, i. e., the or- 

 ganic individual ; Evolution pertains to the 

 modifications of the individual members of 

 a genetic series of successive organisms. 

 These three forms of genetic phenomena 

 are alike in that they all consist in the modi- 

 fication or change in the mode of action or 

 function of the body expressing them. 



In Metabolism, molecules, which in nor- 

 mal chemical phenomena (not organic) have 

 been at rest, or passing into or toward con- 

 ditions of more stable equilibrium, in vital 

 phenomena pass upward into more unstable 

 combinations. I speak, of course, of the an- 

 abolic phenomena of metabolism. 



In Development, bodies, which under the 

 influence of physical forces would move to- 

 ward a state of greatest rest and equilibrium, 

 are in the living body actively engaged in 

 changing position and overcoming resisting 

 forces. 



In Evolution, series of bodies, normally 

 revolving in adjusted cycles of generational 

 reproduction are slowly departing from the 

 hereditary course of these cycles, and ac- 

 quire new characters which their ancestors 

 did not possess. 



The recognition of the fundamental na- 

 ture of this principle of varying, or trans- 

 mutation, in living bodies not only ties to- 

 gether all the vital phenomena into a 

 consistent system of correlated processes, 

 but it brings their phenomena into a natural 

 relationship to the normal phenomena of 

 inorganic matter. 



The path by which these conclusions are 

 reached is not a new one, but is simply an 

 extension of the same line of thought which 

 a century ago led to the overthrow of the 

 Cuvierian notion of species. The mutability 

 of species was a necessary preliminary step 

 in the formation of a clear notion of organic 

 evolution. "We must carry the idea one 

 step further and recognize the essential 

 mutability of the organism. As in the last cen- 

 tury the whole classification of organisms 

 was based on the theory that the species 

 was an immutable unit, so at the present 

 time the whole classification of biological 

 phenomena is based on the assumption that 

 heredity is a fixed immutable law. The prin- 

 ciple of mutability must be recognized in the 

 phenomena of development before we can 

 rightly comprehend the laws of organic life. 



