INTRODUCTION 3 



are in any way invalidated because they are based on zoo- 

 logical data alone. We are concerned with the evolution of 

 animals and are content to let our conclusions speak for them- 

 selves. It is very probable that there are certain evolutionary 

 principles and phenomena that are peculiar either to animals 

 or to plants. Polyploidy and certain other chromosomal 

 phenomena seem at present to be almost restricted to the 

 latter. We do not, however, believe that the truth or falsity 

 of any theory of evolution is likely to be decided by an 

 acid test provided by exclusively botanical or zoological 

 data. 



The importance of variation in the study of evolution is 

 too well known to require much explanation. Whatever we 

 may hold to be the cause or causes of the evolutionary process, 

 it is almost invariably recognised that it has proceeded by the 

 progressive accumulation of changes of the same dimensions 

 as are found in the variation within a species. In spite of the 

 considerable changes that have taken place in evolutionary 

 inquiry, the fundamental idea enunciated by Darwin and 

 Wallace that evolutionary divergence is the summation of a 

 series of changes having the status of individual differences 

 is still almost universally accepted. Students of evolution are 

 still concerned with the questions — how do such variations 

 arise and by what means are they amplified so as to give 

 progressive change in given directions ? 



Some measure of variation is of universal occurrence among 

 all living organisms, and the capacity to display this phenome- 

 non might be given as one of the attributes of living matter. 

 It is doubtful indeed whether it is an exclusive property of living 

 organisms or even of organic compounds (Reichert, 1919) ; 

 but it is far more marked in them than in inorganic bodies. 



The origin of variation is fully discussed in Chapter II. 

 The most generally accepted view, of course, is that, while 

 the somatic tissues are readily modified by environmental 

 factors, heritable variation is due to spontaneous changes at 

 single loci in the chromosomes (gene- or point-mutations), 

 to various kinds of chromosomal abnormalities, or to the 

 combination of maternal and paternal genes. In certain 

 conditions, however, it seems that mutations may be induced 

 by environmental factors. Whether this is a correct view and 

 whether all heritable variation may not in the last resort be 



