344 Organic Nature s Riddle 



climatic change. But still more strange and striking 

 changes have been recorded as due to external con- 

 ditions. Thus it is said^ that certain branchiopodous 

 creatures of the crab and lobster class (certain Crustacea) 

 have been changed from the form characteristic of one 

 genus (Artemia salina) into that of quite another (Bran- 

 chipus), by having been introduced in large numbers by- 

 accident into very salt water. The latter form is not only 

 larger than the former, but has an additional abdominal 

 segment and a differently formed tail. Such changes tell 

 strongly in favour of the existence in creatures of positive, 

 innate tendencies to change in definite directions under 

 special conditions. 



It is also obvious that the very same influences (e.g. 

 amounts of light, heat, moisture, etc.) will produce different 

 effects in different species, as also that the nature of some 

 species is more stubborn and less prone to variation than 

 that of others. Such, for example, is the case with the 

 ass, the guinea-fowl, and the goose, as compared with the 

 dog, the horse, the domestic fowl, and the pigeon. Thus 

 both the amount and the kind of variability differ in 

 different races, and such constitutional capacities or incapa- 

 cities tend to be inherited by their derivative forms, and 

 so every kind of animal must have its own inherent powers 

 of modifiability or resistance, so that no organism or race 

 of organisms can vary in an absolutely indefinite manner; 

 and if so, then unlimited variabihty must be a thing 

 absolutely impossible. 



The foregoing considerations tend to show that every 

 variation is a fimction^ of ' heredity' and ' external influence ' 

 — i.e. is the result of the reaction of the special nature of 

 each organism upon the stimuH of its environment. 



^ Nature, 1876, June 8, p. 133. Schmankevitsch at Odessa. 

 2 In the mathematical sense of the word. 



