INTRODUCTION TO THE METHOD 



their craft by an extension of the Mendehan experimental methods. The evolutionary- 

 context, however, came back into the picture after the unification of genetics with the 

 sister science of cytology, which occurred in the period between the first two world 

 wars. This was made possible in the first instance by the introduction of team work 

 into genetics, notably by the American school of workers on Drosophila founded by 

 \ Morgan in the 1910's, but the final proof that the chromosomes are the seat of Mendelian 



\ inheritance was first conclusively given in 1931 simultaneously for an animal, Droso- 

 phila (Stern, 1931), and a plant, Z^a mays (Creighton and McChntock, 1931). 



'The resulting establishment of cytogenetics as an exact science must be recognized 

 as one of the biggest intellectual achievements of the first half of the twentieth century. 

 The roots of cytology, as of genetics, can be discerned in the nineteenth century, 

 some well-known landmarks being 1875, the date of the publication of ^ellbildung 

 und ^elltheilung by the botanist Strasburger with its factual description of mitosis,* 

 and 1894, the numerical demonstration of chromosome reduction by the same author. 

 Nevertheless, we know that cytology no less than genetics was a twentieth-century 

 science from the fact that almost everything in Strasburger's 1894 paper was erroneous 

 except the basic numerical conclusion. It required the pioneer work of Gregoire in the 

 years preceding 19 10, Janssens, Belling and others in the 1920's, and many more 

 workers, both earlier and later, to stabilize technique and to elucidate the funda- 

 mental descriptive facts of meiosis * without which cytogenetics could not have been 

 established. 



'By this means as the century advanced, a new tool of great and unexpected power 

 was made available for students of evolution. It became possible to investigate the 

 nature of species, or at least of some species, experimentally, to diagnose their mode 

 of origin and trace with precision some significant parts of their genealogy. The pre- 

 occupation of biologists with tracing phylogeny on the grand scale gave place to the 

 attempt to analyse some actual evolutionary mechanisms by experimental means. 

 The success which attended these efforts was, in the first place, of importance as a 

 direct proof, if proof were needed, that the idea of evolution represented not a theory 

 but a fact, and in the second place to shatter the Darwinian conception as to ways and 

 means. 



'The destructive effects of the new knowledge on the compelling simplicity of 

 "Darwinism" came from the recognition that its apparent simplicity was over- 

 simplification, and that what was next req^uired was not one generalization to account 



* An elementary acquaintance on the part of the reader with the basic facts of mitosis and meiosis 

 is here presupposed, although it is sufficient at this stage to know merely that mitosis is another name for 

 nuclear division of the ordinary kind, while rneiosis is a peculiar form of nuclear division, sometimes 

 referred to as reduction division, which occurs at one point only in the life cycle of every species which 

 reproduces by sexual means. In mitosis the chromosomes split longitudinally and their number remains 

 constant throughout the process. In meiosis there are always two nuclear divisions in rapid succession, 

 in the first of which the chromosomes pair and then separate to opposite poles so that their number 

 is halved in each of the resulting nuclei. A detailed knowledge of the mechanism of meiosis is not 

 required for the purpose of this book, but such parts of it as are essential, notably some details of 

 chromosome pairing, will be described in relation to illustrative examples in Chapters i and 2 and 

 especially Chapter 3. 



i 



