XVI.] THE LAWS OF VAEIATION. 189 



Habits are capable of change; but only a sligbt change 

 is possible in a -short time. 



Changes of external circumstances are beneficial to 

 organisms, if they are slight ; but injurious if they are 

 great, unless made gradually. 



Changes of external circumstances are agreeable to the 

 mind when slight, but disagreeable when great. 



Mixture of different races is beneficial to the vigour of 

 the offspring, if the races mixed are but slightly different ; 

 but very different races will produce either weak offspring, 

 or infertile offspring, or none at all. 



It is a very remarkable fact, that confinement to a small EtFect of 

 area appears to have a tendency to diminish the size of ^^ent to a 

 a race of animals, even when there is no other evidence of small area, 

 diminished health and vigour. Thus, animals found on 

 islands are frequently smaller than those of the same 

 species found on continents ; and animals bred in an 

 aquarium are often observed to degenerate in size, even 

 while the race continues vigorous.^ It is scarcely possible 

 to doubt that such diminution is caused by want of room 

 for sufficient variety in the conditions of life, or by want 

 of sufficient mixture of the race — in other words, by the 

 interbreeding of too small a number of individuals — or 

 most probably by both ; though the second reason will 

 not apply in cases where the diminution is visible in 

 a few generations among species of organisms that are 

 able to propagate for a long time without union of the 

 sexes." 



Wliat may be called the law of sexuality in organisms — Sexuality, 

 that is to say, the necessity of the union of two unlike 

 individuals of the same species for the purpose of genera- 



1 I state these facts on the authority of Mr. (uow Sir John) Lubbock's 

 paper on the reproduction of Daphnia. (Philosophical Transactions, 1857.) 

 The Daphnia is a small fresh-water entomostracous crustacean, 



^ Ability to propagate without union of the sexes is not the same thing 

 with hermaphroditism. The land mollusca (snails and slugs) have both 

 the sexes on the same individual, and yet they cannot fertilize themselves. 

 On the other hand, no winged insect is a hermaphrodite, and yet many 

 species produce fertile eggs without impregnation. 



