396 EVOLUTION 



could reconstruct the genealogical history (phytogeny) of the 

 present-day forms. Those of the past would furnish the links 

 connecting genus with genus, and family with family, till all 

 were connected up to the primitive simple organisms from which 

 they sprang when life first developed. The fact that many fossil 

 plants and animals do combine characters, that now serve to 

 distinguish separate groups, is one of the strongest pieces of 

 evidence for such a conception. But, further, the geographical 

 distribution of living plants and animals, showing, as already 

 noted, restriction of similar species to definite areas, is most 

 readily explained as due to their origin, in that area, from common 

 or closely related parents. 



The features characteristic of the various members of a genus 

 or family are often only fully apparent in the adult state, whilst 

 the earlier the stage of development of an organism, the more 

 difficult does the determination of its identity become. The his- 

 tory of the individual may be regarded as, to a limited extent, 

 recapitulating the history of the race ; in this connection it may be 

 noted that in the ordinary course of reproduction every individual 

 commences life as a single cell. Such an interpretation also 

 explains the frequent occurrence of rudimentary structures (e.g. 

 the gill-slits in the embryo of the Chick, or the trefoil leaves in 

 the seedling stages of species of Acacia possessing phyllodes, 

 Fig. 229), which often perform no function, or are even com- 

 pletely lost, in the adult. Even amongst living organisms a 

 graduated series, as has been seen in Chapters XIII to XXIV, 

 can be recognised. The simplest members of this series are 

 doubtless relics, though almost certainly modified, of the earliest 

 flora and fauna, which have found a place in the economy of 

 nature even under existing conditions. 



The highest efficiency is only attained by great specialisation 

 which proportionately diminishes the capacity for adaptation 

 to a new environment. But the world of living things is a world 

 of never-ceasing change, and hence the past history of the organic 

 universe is the history of extinction of specialised races and 

 individuals. The future of a group is thus seen to be dependent 

 upon its less specialised, and thus at the moment less successful, 

 members. But as in time, so too in space, the spread of a species 

 may be handicapped by its lack of plasticity. 



