196 Inheritance of Acquired Characters 
acting also in this same direction throughout a decidedly 
large number of successive generations. 
“One must admit,” says Hartmann, very rightly, 
“that minute and purely accidental variations even if 
they are useful, are unable to preserve themselves from 
disappearing again through crossing. Whatever is to 
be preserved must, as Darwin also admits, appear in a 
certain quantity, either all at once or successively, because 
the number of similar variations must be sufficient to 
overcome the suppression through crossing. But it is 
not to be expected that similar variations will appear in 
such frequence by chance, but only as a result of definite 
external or internal causes which set a definitely directed 
modification in the place of accidental ones.” 153 
Of the causes of variation which possess this capac- 
ity of simultaneous similar and constant action, we know 
at present only functional adaptation aided by the inher- 
itance of acquired characters. 
The very fixity of many species has been rightly 
urged against Weismann. Natural selection in fact, 
because of the smallness of fortuitous individual vari- 
ations, is forced, on the one side in order to explain by 
itself the development of species, to fall back upon an 
excessively great degree of selective capacity; but on 
the other side if this great degree of selective capacity 
is accorded, it encounters still greater difficulty in account- 
ing for the contrary phenomenon presented by a host of 
other species which have remained unaltered even dur- 
ing a whole series of long geologic periods, 
The Lamarckian theory does not find any special 
*°Eduard von Hartmann: Die Abstammungslehre seit Darwin. 
Annalen der Naturphilosophie, herausg. v. W. Ostwald. Zw. Bd., 
Heft. III. Leipzig, May 26, 1903. P. 280. 
