THE NOTION OF SPECIES 7 



odorafa, the roseal substance of Cicindela, the S(NH4)2 of 

 Phallus and other chemical characteristics are produced. 



§ 8a.— HEREDITY. INDIVIDUAL VARIATION. MU- 

 TATION. — Let us venture to proceed one step further along 

 the way of suggestion. 



Almost all the properties of living beings are variable within 

 the limits of each species. The abiotic chemical properties 

 follow the ordinary rule. We know, for instance, that the 

 composition of opium and many other animal and vegetable 

 chemical products is variable according to external causes 

 (conditions of existence ; for instance, climate). It is therefore 

 possible and even probable that the living mixture of each 

 species is also subjected to variation. It may be suggested 

 that two kinds of variation occur : 



(i) Individual variation (variation properly so called) pro- 

 duced by the diversity of the conditions of existence (external 

 causes) . In this form of variation it may be supposed that the 

 individuals differ quantitatively — i.e. in the proportions in which 

 the biotic components of the living substance are mixed, the 

 qualitative composition (that is to say, the nature of the com- 

 ponents) of the specific mixture being invariable in the species. 



The qualitative composition of each specific mixture is trans- 

 mitted by inheritance, and therefore the possibility of producing 

 the reactions which result in specific properties (characters) is 

 also hereditary. (This does not exclude segregation among the 

 components of the living mixture of a hybrid, in which the 

 components of both parents are brought together into one 

 mixture.) It is also admissible that the quantitative com- 

 position of each individual is hereditary to a certain degree, 

 and, moreover, that quantitative individual peculiarities may 

 be accumulated (increased) under the influence of selection ^ 

 so often as the same conditions of existence prevail for two or 

 several generations, the increase alluded to being confined 

 within certain limits which are characteristic of each species 

 (see § 9). A complete discussion of this question lies outside 

 the scope of the present book. (See § 129.) 



(2) Certain causes, which are probably diverse (§ 105), may 

 produce, in the qualitative composition of the living mixture 

 of certain individuals of a species, a modification which is 

 hereditary and called sport, saltation or mutation. If one of the 

 components disappears, or is transformed into a new chemical 

 entity (for instance, into an isomer), or if one new component is 

 added to those previously existing, a slight mutation takes 

 place, the new living form being, for instance, a pure line (§ 19) 

 or a subspecies (elementary species) little different from the 

 1 1 am alluding here especially to experimental (artificial) selection. 



