136 EXPERIMENTAL EVOLUTION LECT. 



according to Cornevin ; common valerian, when 

 growing in marshes, is half or three-quarters less 

 toxic than when it grows in dry soil, according to 

 Pierlot ; 1 and Coffcea arabica is killed by insect 

 parasites (Cemiostoma caffeohnn] which do not attack 

 C. liberica? 1 Physiological variability is displayed 

 every day among the individuals of the same pro- 

 geny or of the same brood. Human twins are often 

 very dissimilar in regard to physical and mental 

 abilities. Among animals, the same cluster of eggs 

 gives birth to tadpoles which differ considerably as to 

 the epoch at which the tail is cast off and the limbs 

 put in their appearance, although all external con- 

 ditions of temperature, food, etc., are exactly identical. 

 Among hogs, the last born of the brood is generally 

 much weaker than the others ; of the four or five dogs 

 which are generated by the same parents, no two are 

 alike in regard to the acuteness of the smell, swiftness, 

 etc. Young turkeys of the same brood differ much, 

 while there is but little difference between young ducks 

 or geese. In my experiments on Lymnaea stagnates 

 I have often noticed very marked differences in the 

 length and weight of animals from the same cluster of 

 eggs, although they lived together, under exactly 

 identical conditions, in the same aquarium. For in- 



1 Note sur la Valerians. Bull. Soc. Bot. France, vol. ix. p. 189. 



2 Nature, vol. xxiii. 1881, p. 541. 



