28 BULLETIN 66, UNITED STATES NATIONAL MUSEUM. 



the possibility is that the race of host and parasites both will become 

 exterminated, the former for the lack of ability to reproduce, the 

 second because of the extermination of their means of livelihood. 

 Theobald (1892) cites the case of Andrena lapponica, at St. Leonards- 

 on-Sea. The bees were found in four localities, but extremely local 

 in 1886. In 1887 he took 180 specimens of which 105 were sty- 

 lopized. In 1888 they were much scarcer, although the climatic 

 conditions were much the same, and the colonies undisturbed; of 60 

 specimens taken, 54 were badly attacked. In 1889 a few were 

 taken, and in 1890 they had entirely gone. Although the evidence 

 in this case is strongly in favor of the supposition that the stylops 

 was the cause of the extinction of the colony, it must not be over- 

 looked that perhaps Mr. Theobald and other zealous collectors also 

 contributed greatly to the same end. 



Only one other definite record of excessive parasitism is on 

 record — that of Polistes texanus collected at Pecos, Texas, by A. E. 

 Brown on September 25, 1901 (Skinner, 1903, a, b), in which 23 per 

 cent of the wasps were parasitized. Only 17 per cent of the Polistes 

 annularis studied by the author was parasitized. 



The very recent study of Andrena crawfordi, however, has added 

 another important instance under this heading. The extent of 

 parasitism in this species has been cited in the conclusions just 

 preceding and need not be further considered at this point. 



It is of course undeniable that certain influences act upon each 

 other in such a manner as to prevent the extermination of a species 

 under ordinary circumstances. In order to explain this fact Marchal 

 (1897 a) imagines a line or curve of numerical evolution for each 

 species of insect. As the curve of the parasite increases it interferes 

 with the corresponding curve of the host, causing a disturbance of 

 the numerical equilibrium of the latter and consequently upsets the 

 equilibrium of the parasite. Unless some external cause stops the 

 downward curve of the two species by partially arresting the develop- 

 ment of the parasite, the two species become extinct. In a state of 

 nature this downward curve may be either checked or not, and hence 

 the rising and falling curves will continue until there cease to be the 

 proper external factors for retarding the downward curve. In other 

 words Marchal wishes to show that the various organisms of nature 

 are mutually dependent and that extraordinary circumstances will 

 cause the extinction of numerous forms by removing some of the 

 elements which naturally operate with or against each other. 



2. Loss of vitality by host. 



It is of course naturally to be expected, at least in cases where 

 several parasites have attacked one individual, that the host should 

 be greatly enfeebled. Hubbard (1892) in writing of Polistes (amer- 



