90 SECTIONAL ADDRESSES 



in systematics requires a much deeper knowledge of the morphology 

 and bionomics of the animals classified. At the time of Linnaeus and 

 after, when systematics were in their infancy, individual specimens 

 showing marked differences were as a rule diagnosed as representing 

 distinct species, the unit called species being looked upon as essentially 

 a constant. This old view was at the time a new view. With the gradual 

 discovery of the great range of variability exhibited by many organisms, 

 the attitude of the systematist has changed. If formerly distinct-looking 

 specimens found in the same locality had to be proved to belong to the 

 same species before they were accepted as specifically the same, the modern 

 systematist approaches the question from the opposite direction, regarding 

 morphologically similar specimens, whatever their outward appearance 

 may be, as specifically alike until their specific distinctness is established 

 by convincing evidence. This attitude renders research in systematics 

 far more subtle and difficult than it used to be and the results far more 

 reliable. Experience has furnished a guiding principle in the facts that 

 similarity does not necessarily mean relationship of the forms under 

 observation, that dissimilarity is not necessarily evidence of specific 

 distinctness, and that variability obtains in every species and every organ ; 

 and if these facts are kept in mind by the systematist, the reproach of 

 superficiality often justly levelled at work in taxonomy can be borne with 

 equanimity. Variability is an essential character of everything alive. 

 The concept of the constant species of former days is replaced by the 

 concept of the flexible species, and the saying that like breeds like requires 

 modifying into the statement that a population breeds a population with 

 the same extent of variability. If like breeds like were being taken 

 literally, we should have to alter it into like breeds unlike. For, strictly 

 speaking, individuals are never alike whatever their relationship to each 

 other. A calculation, for instance, of the number of specimens required 

 of the commonest British mouse-flea {Ctenophthalmus agyrtes) in order to 

 find among them two absolutely alike in the number and position of the 

 bristles on the body arrives at the amusing figure of many million billions, 

 a figure certainly in excess of that of the whole flea-population of Great 

 Britain, and tantamount to proving that there are no two specimens alike. 

 In spite of all this variability the apparently chaotic mass of organisms is 

 cut up by specific barriers into units represented by populations of 

 numerous individuals, each population living its own life alongside other 

 populations, as anybody can ascertain in his own garden or as a matter 

 of fact in his own flat, particularly if there are a cat and a dog about. 



In studying the characteristics of each specific unit and drawing up 

 diagnoses for purposes of recognition, the systematist renders service in 

 two quite different spheres of work and thought. Being alone able to 

 identify the species in the difficult group in which he specialises, he assists 

 defensive biology in its task to safeguard humanity against the ravages of 

 health- or food-destroying organisms. Applied biology can only be a 

 science if based on sound systematics. You will forgive me, I hope, if 

 I refer, as a case in point, to an instance in the work of my late brother ; 

 it is a story well known to all who are interested in tropical diseases and 

 perhaps a little worn by now, but will always remain a very instructive 



