INTRODUCTORY 5 



developing individual must be thought of as on a curve — a trajectory 

 — with its ascent and its descent alike, often with characteristic 

 "ages" or phases. And again, we must think of each of these phases 

 as having its possibilities of lengthening or shortening in adaptation 

 to particular habits and conditions. All this is discussed in the 

 section of the book that deals with the Development of Organisms. 



PALiEONTOLOGICAL.— But the inquiring student, who has 

 grasped for the individual organism the idea of becoming, cannot 

 but look for its counterpart in the story of each race; in fact, just 

 as we study a Biography in its place in History. The horse was once 

 a foal, and the foal was once a foetus in the womb of its mother; 

 and the story of this is embryological; but the race of horses has had 

 its long-drawn-out evolution, from the small four-toed and three- 

 toed Eohippus of the Eocene meadows to the stately modem 

 Equus running and leaping on the tiptoe of one digit on each limb ; 

 and the story of this is Palaeontological. 



In other words, embryology has to do with individual develop- 

 ment (ontogeny) , and palaeontology has to do with racial evolution 

 (phylogeny). But the two must be correlated, even at the risk of a 

 vicious circle, for the palaeontologist, whose main material consists 

 of fossils, may receive hints from the embryologist, who has some- 

 times been the first to discern relationships; and the embryologist 

 may get light from the palaeontologist, since the individual develop- 

 ment tends to be in some respects an abbreviated recapitulation of 

 the racial evolution. 



Palaeontology describes, as far as possible, the gradations that 

 may lead, among fossils, from one species to another (particularly 

 well illustrated in Planorbis and other fresh-water snails); the 

 connecting links between distinct types (well seen in extinct Ceph- 

 alopods), the affiliation of classes (as of Birds from Dinosaurian 

 Reptiles), and even the origin of a particular association of organ- 

 isms, such as the Deep Sea Fauna or the Carboniferous Forest. 

 Thus there may be a palaeontology of species, types, classes, phyla, 

 and associations. With the last may be included an inquiry into 

 geographical distribution, how different countries have come to be 

 tenanted by similar or dissimilar faunas and floras. In essence, 

 palaeontology is a description of the stages by which organisms have 

 come to be as they are; and it fails of its ambition if it does not 

 disclose something of the grandeur of the historical Ascent of Life. 

 It is this most general aspect of Palaeontology that is emphasised 

 in the outline which this book offers ; hence the title of the palaeonto- 

 logical section — Great Steps in Organic Evolution. 



ETIOLOGICAL.— Huxley appHed the useful term Etiology to 

 the study of the factors that have operated in the process of 



