8 ON THE ORIGIN OF SPECIES 



for many generations. No case is on record of a vari- 

 able being ceasing to be variable under cultivation. 

 Our oldest cultivated plants, such as wheat, still often 

 yield new varieties : our oldest domesticated animals 

 are still capable of rapid improvement or modification. 

 It has been disputed at what period of life the causes 

 of variability, whatever they may be, generally act ; 

 whether during the early or late period of development 

 of the embryo, or at the instant of conception. Geoffroy 

 St. Hilaire's experiments show that unnatural treatment 

 of the embryo causes monstrosities ; and monstrosities 

 cannot be separated by any clear line of distinction 

 from mere variations. But I am strongly inclined to 

 suspect that the most frequent cause of variability may 

 be attributed to the male and female reproductive 

 elements having been affected prior to the act of con- 

 ception. Several reasons make me believe in this ; but 

 the chief one is the remarkable effect which confine- 

 ment or cultivation has on the function of the repro- 

 ductive system ; this system appearing to be far more 

 susceptible than any other part of the organisation, 

 to the action of any change in the conditions of life. 

 Nothing is more easy than to tame an animal, and few 

 things more difficult than to get it to breed freely under 

 confinement, even in the many cases when the male and 

 female unite. How many animals there are which will 

 not breed, though living long under not very close con- 

 finement in their native country ! This is generally 

 attributed to vitiated instincts ; but how many cultivated 

 plants display the utmost vigour, and yet rarely or never 

 seed ! In some few such cases it has been discovered 

 that very trifling changes, such as a little more or less 

 water at some particular period of growth, will determine 

 whether or not the plant sets a seed. I cannot here 

 onter on the copious details which I have collected on 

 this curious subject ; but to show how singular the laws 

 are which determine the reproduction of animals under 

 confinement, I may just mention that carnivorous 

 animals, even from the tropics, breed in this country 

 pretty freely under confinement, with the exception of 



