Later investigators, notably Eimer, Weismann, Mendel 

 and DeVries, have made valuable contributions, amending 

 and enlarging upon the theories proposed by Lamarck and 

 Darwin. 



(c) — Lamarck' and Lamarckism 



Lamarck (1744-1829), in his Philosophie Zoologique, 

 published 1809, expounded more clearly than any of his 

 predecessors a logical explanation of evolution. He recog- 

 nized three factors: 



(1) a Changing Environment: 



(2) the Effect of Use and Disuse; and 



(3) the Inheritance of Acquired Characters; 



He reasoned as follows: Great changes in environ- 

 ment produce in animals new wants; new wants — changed 

 habits; changed habits — the use of new parts or the disuse 

 of old ones; and finally after many generations the pro- 

 duction of new organs and the modification of old ones. 

 He believed, however, that plants are modified directly by 

 their environment. (Fig. 1) 



Lamarck summarized his views in the following laws: 



First Law. — "In every animal which has not finished 

 its term of development, the frequent and sustained use of 

 any organ strengthens and develops it, and increases it in 

 size proportionate to the length of time it has been employ- 

 ed. On the other hand, the continued lack of use of any 

 organ gradually weakens it until, at last, it disappears" 

 (Law of Use and Disuse). 



Second Law. — "Nature preserves everything she has 

 caused the individual to acquire or lose through the influ- 

 ence of the environment to which its race has been for a 

 long time exposed, and hence the predominance or loss of 

 certain organs through the use or disuse. She does this by 

 the production of new individuals which are endowed with 

 the newly acquired organs, provided the acquired changes 

 were common to the two sexes of the individuals that pro- 

 duced the new forms" (Law of Inheritance). 



The essence of Lamarckism is, therefore, adaptation of 

 plants and animals to their environment through function, 

 or through the use or disuse of organs, and the inheritance 



(1) — Lamarck had a varied career. He spent his boyhood at a 

 Jesuit College in Amiens; served in the French army, studied 

 medicine for four years; devoted himself first to Botany, later 

 to Zoolog-y. other publications were: Flora of France (1778), Natural 

 History of Invertfbrated Animals (1815-1822). He reorganized the Jardm des 

 Plantes (1793). 



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