4 BURSA BURSA-PASTORIS AND BURSA HEEGERI : 



cations of the old ones, as may be necessary for the interpretation of the 

 observed facts of evolution. Among- the species now being- studied , one that 

 has very early yielded results of interest in this connection is the common 

 shepherd 's-purse of our dooryards, Bursa bursa-pastoris (L.) Britton, 



Bursa (Capsella} bursa-pastoris is known to the taxonomist as an exceed- 

 ingly variable species, which seems to have been brought to this country from 

 Europe, but which is now naturalized and almost universally distributed 

 throughout the North Temperate zone. So strikingly different, qualita- 

 tively, were the characters of different individuals observed growing side 

 by side in nature that it was difficult to believe that they all belonged to a 

 single series of fluctuating variations, and when the opportunity offered 

 to make pedigree-cultures this species at once sug-gested itself as favorable 

 material. This thought was based not alone upon its apparent polymorph- 

 ism, but also upon its hardiness, ease of culture, and the impossibility of 

 its having been subjected to any of the artificial conditions of isolation, 

 crossing, etc., which are usually thought to render plants of economic 

 value unfit to give information regarding the behavior of plants in nature. 



No one has ever attempted to "improve" the shepherd's-purse, and 

 although its rapid extension over its present great range is undoubtedly 

 dependent upon the agency of man, both in supplying suitable habitats and 

 more directly in the transportation of the seeds, yet in performing these 

 operations his work has been wholly unintentional, and he is therefore to 

 be classed with the other accidental agents of nature, thus leaving to Bursa 

 the solution of its problems of maintenance, extension of range, and evo- 

 lutionary progress, under conditions which are fundamentally like those 

 that must be met by any other species in a state of nature. 



These cultures were begun in April, 1905, and were continued until the 

 spring of 1907, when they were temporarily abandoned because other in- 

 vestigations necessitated my absence from the Station for Experimental 

 Evolution during rather extended periods, and thus made it impossible to 

 continue the work advantageously on a species like Bursa, which shows no 

 dependence upon the seasons, but blooms and fruits whenever external 

 conditions are such as to make its development possible. It is hoped that 

 these studies on Bursa may be continued in the not distant future, but as 

 some of the conclusions arrived at have been already presented before sev- 

 eral scientific bodies,* it seems desirable to publish a more comprehensive 

 account of the work than has been done up to this time, even though the 

 evidence is in many places more or less fragmentary. 



When undertaking such cultures with any new class of material much 

 that is necessary for the most satisfactory and economic conduct of exper- 



*Sections F and G, A. A. A. S., New York, December, 1906; Seventh International 

 Zoological Congress, Boston, August, 1907; Botanical Society of America, Chicago, 

 January, 1908. 



