THE FACTORS OF ORGANIC EVOLUTION. loi 



rather than upon the evidences for the Lamarckian or 

 the Darwinian factors. 



I call attention very briefly to the following propo- 

 sitions : I. Development, and consequently evolution, is 



the result of the interaction of extrin- 

 Statement of ^-^ ^^^ intrinsic causes. 2. Intrinsic 



propositions. , , , , 



causes are dependent upon protoplas- 

 mic structure. 3. Inherited characters must be prede- 

 termined in the structure of the germinal protoplasm. 



4. Germinal, as compared with somatic,* protoplasm is 

 relatively stable and continuous, but not absolutely so, 

 as maintained by Weismann ; therefore, extrinsic causes 

 may modify both germinal and somatic protoplasm. 



5. It is extremely difficult to determine whether or not 

 extrinsic factors have modified the structure of the 

 germinal protoplasm. This is illustrated by some of 

 the evidences advanced for the inherited effects of 

 diminished nutrition, changes in environment, use and 

 disuse. 6. Experiment alone can furnish the crucial 

 tests of these Lamarckian factors. 



I. The causes of development in general are usually 

 recognised as twofold — extrinsic and intrinsic. As ex- 

 amples of extrinsic causes may be men- 

 Causes of de- tioned gravity, surface tension, light, 

 velopment. , . .... , 



heat, moisture, and chemism in general; 



examples of intrinsic causes are the non-exosmosis of 

 salts from living bodies in water, the pouring of a glan- 

 dular secretion or the sap of plants into a cavity under 

 high pressure, the active changes in shape and position 

 on the part of cells, assimilation, growth, division, etc. 

 There is not, however, a uniformly sharp and distinct 

 line of demarcation between these two factors of develop- 



* Somatic cells are those composing the tissues of the body 

 as distinguished from germ cells — those destined to form the new 

 organism. 



