82 SECTIONAL ADDRESSES 



The paleontologist, confronted with his continuous and long-range 

 trends, is prone to misunderstand the implications of a discontinuous 

 theory of change such as mutation, and to invoke orthogenesis or Lamarck- 

 ism as explanatory agencies. Since there exist more rare than abundant 

 species, the biogeographer will have to discount the fact that he is dealing 

 mainly with processes irrelevant to the major trends of evolution regarded 

 as a long-range process ; while the ecologist and the pure physiologist, 

 appalled by the complexity of the phenomena, are apt to give up any 

 quest for evolutionary explanation. 



Selection in a Mendelian World. 



In our attack upon the problem, we must first mention some implica- 

 tions of recent genetics. Essentially, the modern conception may be 

 put as follows. The notion of Mendelian characters has been entirely 

 dropped. Instead of a given gene having a constant effect, its actual 

 effect is dependent upon the co-operative action of a number of other 

 genes. Mutations which in one gene-complex are pathological, in 

 another may be perfectly harmless, and in yet another advantageous. 

 The adjustment of such mutations to the needs of the organism may 

 occur entirely through recombination of existing modifiers, or, after a 

 preliminary and partial buffering by this means, the final adjustment 

 may have to wait upon further mutation. 



Thus evolution need not occur by a series of sharp single steps ; each 

 such step is immediately buffered by ancillary changes in genes and gene- 

 combinations. What evolves is the gene-complex ; and it can do so in 

 a series of small if irregular steps so finely graded as to constitute a con- 

 tinuous ramp. 



When we reflect further that it is theoretically possible for a gene to 

 alter its character radically by mutating step by small step from one 

 multiple allelomorph to another, we shall see that the discontinuity in- 

 herent in Mendelian genetics is no obstacle to the visible continuity 

 revealed in palaeontological evolution. 



Nor is the pathological character of many mutations at their first 

 appearance necessarily a bar to their final evolutionary utilisation by the 

 species. Let us take some examples of this last-named process. The 

 mutant gene eyeless in Drosophila was originally described as considerably 

 reducing the size of the eyes, in some cases to complete absence, markedly 

 decreasing fertility, and depressing viability. When, however, a 

 stock for eyeless was inbred for a number of generations, it was found 

 that practically all had normal eyes and showed little reduction in either 

 fertility or viability. On outcrossing to the normal wild type and re- 

 extracting the recessives in F2, it was found that these once more mani- 

 fested the original characters of eyeless, though in even more variable 

 degree. 



The explanation of these facts is that the manifestations of eyeless 

 are readily influenced by other genes, and that in general those modifiers 

 which make for normal viability and fertility also make for normality 

 in eye-size. Thus natural selection acting upon the recombinations of 

 modifiers present in the stock speedily saw to it that the combinations 



