INTRODUCTION TO THE METHOD 



And what, lastly, are the accumulated effects of each type of evolutionary change 

 likely to be if carried on and repeated over a span of millions of years? Only when some 

 reply has been obtained to this last question shall we be in a position to assess the real 

 power of our present tools and to judge whether or not a generahzed evolutionary 

 theory can in fact be constructed. 



It may be said at once that this stage will not be reached in the course of this book, 

 nor, in the opinion of the writer, is it to be looked for for many years to come. In the 

 meanwhile we may cultivate our garden, but before doing so it may perhaps be of help 

 to the uninformed reader, if such there be, to explain a little more precisely what it is 

 that cytogenetics at its present stage of development can do. 



Genetics alone can contribute much to an understanding of the differences which 

 separate natural units of less than specific rank. It is true that inquiry is much restricted 

 by the difficulty in most cases of getting behind the necessarily vague concept 'genie 

 mutation'. Sometimes we can determine the place on a chromosome where a 'muta- 

 tion' has occurred. In other cases we can measure some statistical facts about its 

 frequency of recurrence and can sometimes alter this frequency by deliberate inter- 

 ference (induced mutations). As a rule we do not know at all what has occurred unless a 

 piece of chromosome large enough to be seen has become lost or misplaced. We are 

 likewise generally ignorant of how the mutation acts to produce its visible effect. The 

 effects can, however, be studied, their distribution in the progeny of crosses analysed 

 and predicted, both under controlled conditions and to some extent in natural popula- 

 tions, and the accumulated knowledge so gained can in favourable cases give us the 

 basis of a numerical idea of the relative complexity of genetical differences which 

 separate one natural form from another. These natural forms are, however, very rarely, 

 if ever, species, and as a general rule, genetics, unaided by cytology, is unable to 

 extend its analysis beyond the level which Goldschmidt has fittingly labelled ' Micro- 

 evolution' to contrast it with ' Macroevolution', on which alone the attention of the 

 older evolutionists was bent. 



Cytology is, however, in somewhat better case. With the knowledge that the chromo- 

 somes are the seat of genetically active materials and that changes in these are the 

 physical basis of evolution, the comparative study of chromosomes, if informative at all, 

 can be used to give evolutionary information of a type which no other morphological 

 detail can supply. 



The comparison of chromosome numbers between related forms can in favourable 

 instances be a conclusive guide to phylogeny. The classic case of Spartina Townsendii 

 is a well-known example. This putative hybrid detected in Southampton Water in 

 1870,* and variously listed by Wallace, Hooker and others as an endemic variety or 

 species, or as an interspecific hybrid of spontaneous local origin, was conclusively 

 proved to be the cross between our native S. stricta and a locally introduced alien from 



* A good general account of the history of the discovery and spread of S. Townsendii on the British 

 and French coasts will be found in Stapf (1927), from which it appears that the first person to propose a 

 hybrid origin for the species was a French botanist, Foucaud, in 1894. Other early references to the 

 plant are Wallace's Island Life, 2nd ed. (1892), and Hooker's Student's Flora. Huskins (1931) contains the 

 cytological facts and final diagnosis. 



