« 400 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 Delbceuf’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. J/ 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 zlicu, shows us the necessary and sufficient 
condition under which a variety or species originates 
without the aid of any secondary factor.”’ 
Semper,t an eminent zodlogist and morphologist, 
* Revue Scientifique, xix. (1877), p. 669. Quoted by Giard in Rev. 
Sct., 1889, p. 646. 
+ Animal Life as Affected by the Natural Conditions of Existence. 
By Karl Semper. The International Scientific Series. New York, 
1881. 
