REPRODUCTION AND SEX 475 



sion a number of undesirable "recessive" characters, hidden in 

 conditions of exogamy by their "dominant" counterparts. 



Yet this exposure oi undesirable features may be utilised by the 

 breeder for the useful purgation of the herd. 



On the other hand, there is no doubt that when the same unde- 

 sirable feature occurs on both sides of the house, inbreeding tends 

 to diffuse and exaggerate it. 



The value of exogamy or outbreeding is twofold. It provokes 

 variations and thus increases the range of raw material on which 

 selective agencies can work. It also promotes "hybrid vigour" by 

 the pooling of diverse hereditary resources of good quality. The 

 crossing also makes it more likely that a minus on one side will be 

 made good by a plus on the other, or that desirable dominants will 

 strengthen one another's hands. 



When we take into account such evidence as there is from animals 

 and from plants, and such studies as those of Huth {Marriage of 

 Near Kin, 1887), and the instances and counter- instances of com- 

 munities with a high degree of consanguinity, we are led to the 

 conclusion that the prejudices and laws of many peoples against 

 the marriage of near kin rest on a basis not so much biological as 

 social. 



In regard to the marriage of full cousins, the likelihood of 

 unhealthy offspring will be very great if there are the same hereditary 

 taints in the lineage of both parents. If there is a well-defined 

 family predisposition to certain diseases, the fact that the marrying 

 cousins are themselves somatically healthy does not justify them in 

 becoming parents. If two somatically healthy cousins belonging to a 

 tainted lineage have what the Mendelians call a single or simplex 

 dose of the taint, the probability is that on the average one-quarter 

 of their children will be similarly tainted. On the other hand, if 

 the family history is good on both sides, there is no biological 

 reason why two healthy cousins, who fall in love with one another, 

 should not marry joyously, or why they should not have wholesome 

 children. 



Lunar Periodicity in Spawning. — In ancient Greece and Rome 

 there was a general belief, still lingering locally among Mediterranean 

 fishermen, that various marine animals, such as crabs and sea- 

 urchins, are "full" at full moon and "empty" at new moon. This 

 interesting belief has been recently tested by Mr. H. Munro Fox. 

 He finds that the belief is based on fact so far as the sea-urchins of 

 the Red Sea are concerned, but that it does not hold at all for those 

 in the Mediterranean. In neither sea does it hold for crabs or for 

 mussels. 



The reproductive cycle of a Suez sea-urchin {Centr echinus setosus) 

 shows marked lunar periodicity. At each full moon during the 



