4o6 LAMARCK, HIS LIFE AND WORK 



the other hand, the use or disuse of organs is a 

 direct cause of variation, and can furnish natural 

 selection with abundance of material to work upon " 

 (p. 49). The book, like the papers of Allen, Ridg- 

 way, Gulick, and others, shows the value of isola- 

 tion or segregation in special areas as a factor in the 

 origination of varieties and species, the result being 

 the prevention of interbreeding, which would other- 

 wise swamp the incipient varieties. 

 Here might be cited Delboeuf's law:* 



" When a modification is produced in a very small 

 number of individuals, this modification, even were 

 it advantageous, would be destroyed by heredity, as 

 the favored individuals would be obliged to unite 

 with the unmodified individuals. // nen est rien, 

 cependant. However great may be the number of 

 forms similar to it, and however small may be the 

 number of dissimilar individuals which would give 

 rise to an isolated individual, we can always, while 

 admitting that the different generations are propa- 

 gated under the same conditions, meet with a num- 

 ber of generations at the end of which the sum total 

 of the modified individuals will surpass that of the 

 unmodified individuals." Giard adds that this law 

 is capable of mathematical demonstration. " Thus 

 the continuity or even the periodicity of action of 

 a primary factor, such, for example, as a variation of 

 the milieu, shows us the necessary and sufificient 

 condition under which a variety or species originates 

 without the aid of any secondary factor." 



Semper, f an eminent zoologist and morphologist, 



* Revue Scientifique, xix. (1877), p. 66g. Quoted by Giard in Rev. 

 Set., i88g, p. 646. 



f Animal Life as Affected by the Natural Conditions of Existence. 

 By Karl Semper. The International Scientific Series. New York, 



i88i. 



