72 BIOLOGICAL SOCIETY OP WASHINGTON. 



of primary barriers on the distribution of species, as shown in the 

 marked divergence of the faunas on the eastern and western slopes 

 of the Cordillera. "We ought not," he remarks, "to expect any 

 closer similarity between the organic beings on the opposite sides 

 of great mountain ranges than on the opposite shores of the ocean, 

 except for species which have been able to cross the barrier, whether 

 of rock or salt water." (Ibid, pp. 326-7.) 



I believe he was the first to draw attention to the paucity of in- 

 sects on islands, and to establish the principle that the smaller the 

 area, the less favorable it is for the development of insect life. 

 {Ibid, p. 391.) 



It is a fact of observation that islands predispose to the apterous 

 condition among insects, a fact that is especially noticeable in 

 Kerguelen's Land, as observed by Dr. Hooker, and particularly by 

 our fellow member, Dr. Kidder. Darwin (Origin of Species, etc., 

 p. 109,) first suggested the most plausible reason, viz: that the in- 

 discriminate use of wings might prove injurious to an insular 

 species by tempting it out to sea and to destruction, so that the loss 

 of the power of flight is a positive advantage to the species. The 

 argument against this explanation, viz : that insular species should 

 be gifted with strong powers of flight to fortify themselves against 

 being blown to sea in heavy gales, has little force, because either 

 requirement may be fulfilled ; and, in reality, where flight is abso- 

 lutely necessary, as in the majority of Lepidoptera, and flower- 

 frequenting Coleoptera, the wing capacity, in insular species, is 

 actually increased, or correlated with a diminution of bulk ; 

 whereas, in those less dependent on aerial progression, natural se- 

 lection would decrease wing-power, and there would be just such 

 a correlated increase of bulk as is generally the case. 



The principle he laid down, that the accidental introduction of 

 organic beings amongst others to whose interest they are hostile, 

 may be a powerful means of keeping the latter in check, and of 

 finally destroying them, finds vivid exemplification in insects, as I 

 have shown in discussing those imported into this country.* 



* Second Annual Rep. on the Insects of Missouri, 1879, pp. 8-13. 



