Factors in Evolution 23 



duplicate of the parent, and we know that it always 

 departs more or less from the parental type. These 

 departures from the type may be very marked, 

 and it is not unlikely that these changes may be 

 so great sometimes as to pass beyond the 

 limits of the so-called fluctuating variations that 

 are common to all species, and in such cases 

 there arise so-called " mutations," which may 

 be permanent in case crossing is prevented. Such 

 sudden or discontinuous variations are assumed by 

 some biologists to be the all-important cause of the 

 formation of new species. This view is especially 

 held by De Vries, whose studies in mutation have 

 lately attracted so much attention. As to the causes 

 of these mutations, however, we are very much in 

 the dark, and much more evidence is needed before 

 it will be safe to assume that mutations alone are 

 the real origins of new species. 



That the constitution of all the germ cells of a 

 given species is essentially the same, and that in the 

 normal course of development the growing organism 

 is subject to the same conditions, will account for 

 the main facts of heredity, without assuming any 

 special germ-plasm or " formative materials " 

 corresponding to special organs. The phenomena 

 of regeneration in plants all point to the correctness 

 of this view. 



There must be inevitably a greater or less differ- 

 ence between the cells resulting from any cell di- 

 vision, and these differences must be reflected in the 



