258 BIOLOGY: GENERAL AND MEDICAL 



ancestors of the forms now living. The principles to 

 which he adhered were: "1. The great length of geologi- 

 cal time; 2. The continuous existence of organic life 

 throughout all the geological periods; 3. The general 

 similarity of the physical environment throughout the 

 periods; 4. Continual gradual changes without cata- 

 clysms or catastrophic destruction of life; 5. Gradual 

 modifications in the living things to correspond with 

 the environment; 6. Gradual modifications of the habits 

 to coincide with the gradual changes in structure." 



Lamarck believed that all living things arose from 

 germs that developed spontaneously, and that the most 

 simple of these was " monad-like." The first germs of 

 animal and vegetable life were formed in favorable places 

 and under favorable conditions. The functions of life 

 beginning and an organic movement being established, 

 these germs "necessarily gradually developed into 

 organs so that after a time and under suitable circum- 

 stances they have been differentiated" into different 

 parts or organs, development proceeding from the 

 simple to the complex. The time during which such 

 differentiations have been in progress is " absolutely 

 beyond the power of man to appreciate in an adequate 

 way," but "With the aid of sufficient time, of circum- 

 stances which have necessarily been favorable, of changes 

 of condition that every part of the earth's surface has 

 successively undergone in a word, by the power which 

 new situations and new habits have of modifying the 

 organs of living beings all those which now exist have 

 been gradually formed as we now see them." 



In the progress of evolution certain factors were 

 looked upon as essential; these are now known as La- 

 marc kian factors and are: 



1. "Favorable circumstances attending changes of environment, 

 soil, food, temperature, etc., supposed to act directly in the case 

 of plants, indirectly in the case of animals and man. 



2. " Needs, new physical wants or necessities induced by the 

 changed conditions of life. Lamarck believed that change of 

 habits may lead to the origination or modification of organs; that 



