768 



SCIENCE. 



[N. S. Vol. XV. No. 385. 



considerations, that during the increase of 

 a species both the absolute and relative 

 number of varying individuals as well as 

 the amplitude of variation are increased 

 beyond what is usual." 



The opponents of natural selection have 

 often implied that conclusions drawn from 

 domesticated animals cannot be used to 

 explain the origin of varietal and specific 

 forms among animals living under natural 

 conditions. These authors seem tacitly to 

 assume that the conditions of domestication 

 are unique in that they can be realized only 

 in human surroundings and under con- 

 scious human control. This, however, is not 

 the case. The social Hymenoptera among 

 insects, notably the ants, not only exhibit 

 a form of domestication, as I shall endeavor 

 to show, but also a pronounced and regular 

 periodic increase in numbers. Thus they 

 combine both the conditions for producing 

 high variability, and furnish a brilliant 

 illustration of the evolutionary factor to 

 which Adlerz has called attention. Adlerz, 

 who is well known as a myrmecologist, has, 

 of course, utilized the ants to some extent 

 in support of his views. He calls attention 

 to the effects of feeding on the personnel of 

 growing colonies. I believe it is possible in 

 this connection to lay stiU greater stress on 

 the facts. A considerable number of ob- 

 servations on colonies of different species 

 of Pheidole have led me during the past 

 three years to essentially the same views as 

 Adlerz. As these ants beautifully illustrate 

 the union of domestication and numerical 

 increase and a concomitant high degree 

 of variability, it seems best to record 

 some of my observations and reflections in 

 the hope that they may be of interest to 

 those who are undertaking studies in varia- 

 tion. 



Tt is now a well-established fact that 

 every ant-colony is founded by a single 

 fertilized female, or queen. The insect loses 

 her wings and buries herself in a small 



cavity in the soil or wood that is to form 

 the future nest. After entering the cavity 

 she usually closes the opening so that she 

 is completely shut off from the outside 

 world. She deposits, at the expiration of a 

 certain time, a number of eggs, and when 

 these hatch as larvae she does not go abroad 

 in quest of food but feeds her offspring 

 with substances regurgitated frOm her own 

 body. These substances are ultimately 

 derived from the fat body, a store of 

 nutriment accumulated during her life in 

 the maternal nest which she forsook to 

 take the nuptial flight. Of course, the 

 insect must derive her own nourishment 

 from the same internal source and, as 

 in all ants, the development of the young 

 extends over a considerable period of 

 time, it follows that the larvae are of 

 necessity poorly fed and after pupation 

 hatched as dwarf workers (microergates). 

 The number, too, of these diminutive crea- 

 tures is limited, so that the whole colony 

 in this incipient stage is a family consist- 

 ing only of the huge mother and a few 

 dwarf offspring. These workers, though 

 very timid, forthwith break through the 

 walls of the chamber and establish relations 

 with the outside world, whence they bring 

 food into the nest and feed their half- 

 starved parent. This food soon enables her 

 to lay another batch of eggs, the larvae from 

 which are now turned over to the care of the 

 workers. Being better fed, the second litter 

 are able to reach a greater size before 

 pupation and therefore give rise to larger 

 workers than their nurses. The number of 

 workers thus reinforced soon brings about 

 a condition of affluence in the colony. The 

 queen is more and more abundantly fed, 

 and this, coupled with her confined and 

 sedentary life, enhances her fecundity. 

 The colony waxes strong in numbers and 

 the workers of successive batches grow 

 larger till they attain to the full stature of 

 the species. Then, and not till then, do the 



