472 



THE AMERICAN NATURALIST 



[Vol. LIII 



ferences are now discernible from place to place in North Amer- 

 ica, in so far as perfectly comparable material is at hand. This 

 accords with the findings of Phillips (1915), which also were 

 practically negative. 



Are we not to infer, then, that there has not as yet been suffi- 

 cient time (up to three years and as many possible generations 

 in Death Valley and up to sixty years elsewhere in North Amer- 

 ica) for the impress of diverse environments in the different 

 parts of the territory newly occupied by Passer domesHcus hos- 

 tilis to bring physical changes in the birds of sufficient magni- 

 tude for the modern systematist to detect ? Is there not here a 

 demonstration of the relative permanence of subspecific charac- 

 ters far beyond what many naturalists have supposed ? Are not 

 such characters in general far more likely to be germinal than 

 somatic ? 



How intensely interesting it will be to waterh the course of this 

 "experiment," now under way, irrespective of human effort, in 

 Death Valley, with "controls" vigorously maintaining them- 

 selves (against man's wish !) in San Diego, Berkeley and Boston. 



But perhaps it will be urged that the conditions of an orthodox 

 experiment are not here properly met. The "factors" of the 

 environment are not sorted out, and none is under any kind of 

 regulation. Moreover, rigid control has not been secured, in 

 that there is no way in which any of the naturally established 

 colonies of English sparrows can be strictly isolated and kept 

 from genetic contamination by new influxes of birds from else- 



In reply, I would say that we are not expecting more from our 

 natural experiment than the demonstration of what we set out 

 to prove, namely, the length of time necessary for the develop- 

 ment, in a stock under natural conditions some of which are 

 known, of characters of subspecific value. In the breeding cage 

 there are always "unknown" factors; so let us admit the exist- 

 ence of tlio^o ill tlie wild as not invalidating the "experiment" 

 as Mii'li. In ii.ilui-e. subspecies have differentiated under just 

 rli.' .■orHiliions scit'-iiiiposed by the English sparrows through 

 I ndiv id luil song sparrows &nd horned 



