30 THE GROWTH OF GROUPS 



It is well known that captive animals are frequently 

 sterile. A stock which is fertile for a time may become 

 infertile. It is usual to explain the fact as due to in- 

 breeding, but the actual cause of the infertility is seldom 

 known. Certainly there are other causes of infertility. 

 For example, the fertility of captive animals often depends 

 on space. Wild rats will seldom produce young when 

 confined in small cages, but breed readily enough in a 

 large enclosure. A pair of pigeons will produce no 

 young in a small cage. The same pair, however, if 

 removed to a large cage will breed, but the group of 

 their descendants will cease to expand as soon as the 

 cage becomes crowded, the numerical size of the group 

 depending on the capacity of the enclosure. It is not 

 suggested that the deterioration noticed by Ritzema-Boz 

 was due to overcrowding but it is suggested that want 

 of space is in itself a direct cause of infertility, that there 

 must be many other unknown causes, and that one or other 

 will intervene sooner or later in any line of inbred stock 

 under artificial conditions. The fact that inbreeding 

 was carried on for twenty generations before deterioration 

 occurred is remarkable, but it merely proves that the 

 event was possible. In another experiment under other 

 unknown conditions, degeneration might have set in 

 after the fourth or fifth generation ; this would not have 

 proved that inbreeding is harmful to rats in general 

 after the fifth generation. Wisemann, experimenting 

 with mice, also noticed degeneration, but not until after 

 the thirtieth generation. We may be sure that, sooner 

 or later, deterioration will occur among inbred domestic 

 stock, but we do not know that such is due to inbreeding in 

 every case. The question depends on how far debilitated 



