SPECIAL TECHNIQUES 91 



A chain of evidence is no stronger than its weakest Hnk. 

 Precise methods of analysis can be appUed effectively only 

 when the nature of the problem is critically understood. In 

 dealing with anything so complicated as hybridization under 

 natural conditions, we need a quick method for roughing out 

 the problem. To take an actual instance, the employment 

 of this method in the field demonstrated effectively that what 

 at first sight appeared to be a large, more or less freely inter- 

 breeding hybrid swarm was instead a series of highly localized 

 populations each with its own micro-environment and its 

 owTi direction of selection. Until our understanding of the 

 dynamics of vegetation is much more precise than it is at 

 present, w^e shall need simple, diagnostic field methods for 

 summarizing in populations variation trends that are too 

 complex for the unaided mind to grasp efficiently. 



STANDARDIZED PHOTOGRAPHS 



The invention of the miniature camera has made it pos- 

 sible to take large numbers of photographs at minimum ex- 

 pense. Properly standardized, such photographs become an 

 efficient record of population variation, but they have been 

 little used. Their earliest employment was by A. J. Wilmott 

 of the British Museum in his studies of population differ- 

 ences in Salicornia. To date, their only published demon- 

 stration has been in Erickson's studies of Camassia (1941) 

 and in the studies of maize from this laboratory (Anderson, 

 1947; Brown and Anderson, 1947), but they have been used 

 extensively in various laboratories for population analysis on 

 a variety of material. 



Though it is a basically simple technique, it can be given 

 greater precision. The first point to be borne in mind is that 

 standardized photographs are something more than just 

 photographs. They are exact, standardized records and 

 need to be made in as routine a fashion as possible. Since 

 large numbers of them will be very much alike, it is an 

 absolute necessity to photograph the title on each picture ^ near 



