Sl% SECTIONAL ADDBESSES. 



suggestive book on 'Growth, and Form.' These studies, however, 

 have usually considered the structure of an animal as an isolated 

 machine. We have to realise that an organism should be studied in 

 relation to the whole of its environment, and here form comes in as 

 distinct from structm^e. That mode of expression, though loose and 

 purely relative, will be generally understood. By ' form ' one means 

 those adaptations to the surrounding medium, to food, to the mode of 

 motion, and so forth, which may vary with outer conditions while the 

 fundamental structure persists. Though all structures may, conceiv- 

 ably, have originated as such adaptations, those which we call ' form ' 

 arfe, as a rule, of later origin. Similar adaptive fonns are found in 

 organisms of diverse structure, and produce those similarities which we 

 know as ' convergence. ' To take but one simple instance from the 

 relations of organisms to gravity. A stalked echinoderm naturally 

 grows upright, like a flower, with radiate symmetry. But in the late 

 Ordovician and in Silurian rocks are many in which the body has a 

 ciiriously flattened leaf-like shape, in which the two faces are distinct, 

 but the two sides alike, and in which this effect is often enhanced by 

 paired outgi-owths corresponding in shape if not in structure. Expan- 

 sion of this kind implies a position parallel to the eai-th's surface, i.e. 

 at right angles to gravity. The leaf-like form and the balancers are 

 adaptations to this unusual position. Recognition of this enables us to 

 intei'pret the peculiar features of each genus, to separate the adaptive 

 form from the modified structure, and to perceive that many genera 

 outwardly similar are really of quite different origin. 



Until we understand the principles governing these and other adapta- 

 tions — irrespective of the systematic position of the creatures in wliich 

 they appear — we cannot make adequate reconstructions of our fossils, 

 we cannot draw correct inferences as to their mode of life, and we cannot 

 distinguish the adaptive from the fundamental characters. No do\:bt 

 many of us, whether palaeontologists or neontologists, have long recog- 

 nised the truth in a general way, and have attempted to describe our 

 material — whether in stone or in alcohol — as living creatures ; and not 

 as isolated specimens but as integral portions of a mobile world. It is, 

 however, chiefly to Louis Dollo that we owe the suggestion and the 

 example of approaching animals primarily from the side of the environ- 

 ment, and of studying adaptations as such. The analysis of adaptations 

 in those cases where the stimulus can be recognised and correlated with 

 its reaction (as in progression through different media or over different 

 surfaces) affords sure ground for inferences concerning similar forms of 

 whose life-conditions we are ignorant. Thus Othenio Abel ("1916) has 

 analysed the evidence as to the living squids and cuttle-fish and has 

 applied it to the belemnites and allied fossils with novel and interesting 

 results. But from such analyses there have been drawn wider con- 

 clusions pointing to further extension of the study. It was soon seen 

 that adaptations did not come to perfection all at once, but that har- 

 monisation was gradual, and that some species had progressed further 

 than others. But it by no means follows that these represent chains of 

 descent. The adaptations of all the organs must be considered, and one 

 seriation checked by another. Tims in 1890, in sketching the probable 



