THE FAGTORS OF ORGANIC EVOLUTION. jo, 
rather than upon the evidences for the Lamarckian or 
the Darwinian factors. 
I call attention very briefly to the following propo- 
sitions: 1. Development, and consequently evolution, is 
the result of the interaction of extrin- 
sic and intrinsic causes. 2. Intrinsic 
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. 
1. The causes of development in general are usually 
recognised as twofold—extrinsic and intrinsic. As ex- 
amples of extrinsic causes may be men- 
tioned gravity, surface tension, light, 
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- 
Statement of 
propositions, 
Causes of de- 
velopment. 
* Somatic cells are those composing the tissues of the body 
as distinguished from germ cells—those destined to form the new 
organism. 
