BIOCHEMISTRY AND TAXONOMY 323 



a carrier. During the periods of rest, the animal stops breathing. The 

 ox}^gen is not replenished from the water in the intestine and consequently 

 the Pq_^ of the coelomic plasma tends to fall, thus bringing about a liberation 

 of oxygen from the haemoglobin because of the particular position of the 

 dissociation curve. 



II. DIVERSITY WITHIN SPECIES 

 To define a species as consisting of groups of individuals with more or 

 less similar gene combinations m.eans that there are genotype differences 

 within the species itself. In the human species, which has a high variability, 

 we may say that there are no two individuals, even twins, possessing the 

 same assortment of genes. A large amount of data is available on individual 

 variation in humans (see R. J. Williams, 1956). The water concentration in 

 18 normal men and 11 normal women was found to vary from 45*6% to 

 70*2%. We also have a number of results on the quantitative pattern of 

 various enzyme systems. The plasma alkaline phosphatase concentration, 

 is a characteristic of the individual and remains constant, but from one 

 individual to another it was found to vary between 1*29 and 14-0 units 

 (of 600 subjects examined by Clark and Beck). The cholinesterase of red 

 blood corpuscles is constant for a given individual but varies considerably 

 from one person to another, etc. 



III. DIVERSITY BETWEEN SPECIES 

 It is to Aristotle that we owe the invention of the first practical system 

 for the classification of living beings. More than 2000 years ago this natura- 

 list of genius proposed to classify organisms according to the degree of 

 similarity of their morphological and anatomical characteristics. Despite 

 the appearance of other systems claimed to be more natural, but today 

 forgotten, the system of Aristotle has survived and is referred to today as 

 "the natural system". A certain number of species are collected together 

 into a new group, defined by the possession of common characteristics, 

 and several of these groups together form a more general category, etc. 

 By the accumulation of taxonomic data it was possible to formulate a 

 principle which was the subject of much admiration and satisfaction among 

 naturalists; this was, that classification, based upon a few diagnostic 

 characteristics, of a species and the placing of it in a given class enables one 

 to forecast the existence of a whole range of morphological and biological 

 traits in individuals of that same species. This idea, to which we are 

 accustomed today and which appears self-evident and banal, must have 

 stirred those few who glimpsed the possibilities which at that time were still 

 unsuspected by the majority. When finally the merit of this system was 

 admitted, it was recognized that an essential criterion was the existence of 

 a fundamental similarity and not merely a superficial one it also led to the 

 recognition of a common basic plan in the chief natural groups. During 



