THE UNIQUENESS OF THE INDIVIDUAL 



intermediate sizes before it attains to adult weight or stature, 

 so it must traverse a spectrum of intermediate conformations 

 before it can reach its adult shape. A particular shape can only 

 be '■fixed*' if growth itself comes to a standstill or if the differ- 

 ential growth-ratio settles down to unity, so that later growth 

 entails symmetrical enlargement. The size and shape of an 

 animal must therefore be nicely correlated. The advantages of 

 being larger may be offset by unwieldy or otherwise inept pro- 

 portions, and as far as different sizes or shapes may offer 

 competing inducements, so far must they come to terms. It may 

 not be a coincidence that those fish which, of all animals, change 

 their proportion least in development are just those which 

 grow without any known upper limit to their size. 



D''Arcy Thompson''s assay of transformations is made pretty 

 well self evident by the grids superimposed upon the neigh- 

 bouring figures. The somewhat arbitrary tailoring of space 

 which it makes use of is fraught with metaphysical implications, 

 but we must be content to observe that it has a forthright 

 visual appeal, D''Arcy Thompson always compared the adult 

 forms of the members of related genera or species. He com- 

 pared, then, the final products of two separate processes of 

 transformation, instead of comparing the two developmental 

 processes themselves. Ideally he should have put both pro- 

 cesses into cinematic motion, giving us two films of develop- 

 ment instead of two lantern slides taken from the ends. He 

 could thus have given precision to the belief that the rate of 

 change of shape of animals in development, like their specific 

 growth rate, progressively slows down. It is, of course, a 

 generalization that is ''intuitively' obvious — a human embryo 

 changes its shape more rapidly in a month than a child does in 

 a year — but intuitive judgements are inoffensive only when 

 everyone agrees with them, and in palaeontology, where the 

 problem of the assay of form is ever with us, this is by no means 

 so. But D'Arcy Thompson's method as it stands leads to the 



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