20 GENETICS AND EUGENICS 



2. Lamarck regarded new physical needs as a second factor 

 or cause of variations. He supposed that the need of an 

 organ caused the organ to be produced, that need of horns to 

 fight with or of teeth to chew with would cause the produc- 

 tion of horns and teeth respectively. Darwin never adopted 

 this view. 



3. A third Lamarckian factor however Darwin did regard 

 as a genuine cause of variation, viz., use and disuse. The use 

 of an organ, as the arm or leg, causes it to increase in size 

 and strength; conversely disuse causes decrease in size and 

 efficiency. 



4. Inheritance of acquired characters. As regards heredity, 

 Lamarck believed that variations of every sort are inherited. 

 Those which result from direct action of the environment or 

 from use and disuse, we now call acquired characters, and 

 Lamarck supposed that acquired characters are inherited. In- 

 deed he supposed that all variations are of this nature. Dar- 

 win shared Lamarck's view in part; he too probably did not 

 clearly distinguish between variations which we should class 

 as acquired characters and those of other sorts. Certainly 

 Lamarck did not make this distinction, for on his view all 

 variations are what we should call acquired. 



In illustration of Lamarck's views concerning the causes of 

 variations and of consequent evolution, it may be well to 

 quote a few passages largely in his own words, as given in 

 translation in Osborn, pp. 164-171. 



In considering the natural order of animals, the very positive gradation 

 which exists in their structure, organization, and in the number as well as 

 in the perfection of their faculties, is very far removed from being a new 

 truth, because the Greeks themselves fully perceived it; but they were un- 

 able to expose the principles and the proofs of this evolution, because they 

 lacked the knowledge necessary to establish it. In consideration of this 

 gradation of life, there are only two conclusions which face us as to its ori- 

 gin : — The conclusion adopted up to today : Nature (or its Author) in cre- 

 ating animals has foreseen all possible sorts of circumstances in which they 

 would be destined to live, and has given to each species a constant organi- 

 zation, as well as a form determined and invariable in its parts, which forces 

 each species to live in the places and climates where it is found, and there 

 to preserve the habits which we know belong to it. My personal conclusion: 

 Nature, in producing successively all the species of animals, and commenc- 



