de Beer • The Darwin-Wallace Centenary 



279 



snakes, the wings of ostriches and pen- 

 guins, and the flowers of the feather- 

 hvacinth. 



Knowledge of the fossil record in 

 Darwin's time was so imperfect that 

 nothing was then available in the way 

 of series illustrating the course of evo- 

 lution. Nevertheless, he noticed that in 

 Tertiary strata the lower the horizon 

 the fewer fossils there were belonging 

 to species alive today. Paleontolog)' 

 therefore showed that new species had 

 appeared and old species become ex- 

 tinct, not all at the same time, but in 

 succession and gradually. 



Darwin also investigated the prob- 

 lem of interspecific sterility and saw 

 that it was by no means absolute, be- 

 cause numerous examples can be found 

 of different species that produce hy- 

 brids, and in some cases these hybrids 

 are themselves fertile. From the point 

 of view of breeding, therefore, such 

 species behave like varieties. Why, 

 then, can species not have originated 

 as varieties, by descent and modifica- 

 tion from other species? 



From the evidence provided by all 

 these sources Darwin built up an ir- 

 refutable argument that species have 

 changed and originated from other 

 species and that evolution has oc- 

 curred. That he should have been able 

 to do so from such few data is a mark 

 of genius, for at the time when he 

 worked out his conclusions, none of 

 the cases had been discovered which 

 would now be used as the most striking 

 examples with which to illustrate the 

 fact and the course of evolution. Chief 

 among these are the beautiful series of 

 fossils which reveal the evolution of 

 the ammonites or of the horses, step 

 by step, and those which represent the 

 precursors of the various classes and 

 groups of vertebrates such as Arch- 

 aeopteryx or Pithecanthropus. 



The main steps in Darwin's proof 

 of the fact of evolution were estab- 

 lished by 1842, when he committed 



them to paper in the form of a Sketch 

 which he expanded into an Essay in 

 1844, though neither was published by 

 him. Soon after this, another naturalist, 

 Alfred Russel Wallace, was led to ex- 

 plore similar lines of research. 



Wallace's observations were based 

 on the facts, first, that large systematic 

 groups such as classes and orders are 

 usually distributed over the whole of 

 the earth, whereas groups of low sys- 

 tematic value such as families, genera, 

 and species frequently have a very small 

 localized distribution. Second, "When 

 a group is confined to one district, and 

 is rich in species, it is almost invariably 

 the case that the most closely allied 

 species are found in the same locality 

 or in closely adjoining localities, and 

 that therefore the natural sequence of 

 the species by affinity is also geograph- 

 ical." Third, in the fossil record 

 large groups extend through several 

 geological formations, and "No group 

 or species has come into existence 

 twice." 



The conclusion which Wallace 

 drew from these observations was that 

 "Every species has come into existence 

 coincident both in space and time with 

 a pre-existing closely allied species." 



So much of the credit for the estab- 

 lishment of the fact of evolution has, 

 rightly, been accorded to Darwin that 

 it is only just that Wallace's contribu- 

 tion to this problem should be recog- 

 nized and honored. 



THE MECHANISM OF 

 NATURAL SELECTION 



Although Darwin already knew in 

 1837 that evolution was an inescap- 

 able conclusion to be drawn from the 

 evidence, he did not allow himself to 

 proceed any further with his discovery 

 until he had found an explanation of 

 the fact of adaptation. In a general 

 way, all plants and animals are adapted 

 to their environment, for otheuvise 



