viii PREFACE 



particular interpretations. A few experimental results of 

 my own are given, but only where the observations on which 

 they are based are already published and available for 

 reference. 



It will be seen that I have assumed in the following 

 pages a certain continuity of individuality with regard to 

 the chromosomes. In some insects, during the pupal stage, 

 the cells appear to pass into a condition of hopeless con- 

 fusion. It is not possible to distinguish the various organs 

 of the animal's body ; yet it would seem probable, unless 

 evidence be forthcoming to the contrary, that the cells or 

 the protoplasm derived from the cells forming the various 

 organs in the larva, go to form the corresponding organs 

 in the imago. We know that this is what happens in 

 other insects where the apparent disintegration referred 

 to above does not occur. In some organisms the chromo- 

 somes visibly retain their identity from generation to 

 generation of cells, though in many organisms they 

 appear only in connection with the process of division. In 

 the fear that it may be held that I have assumed too much 

 for the chromosomes, I would point out that as a matter 

 of fact I have not assumed more for them than is assumed 

 for the protoplasm forming the cells of the organs of the 

 insects referred to in the example quoted above. 



Cytology is unfortunately almost a foreign subject to 

 some biologists, and I therefore attempted to make the 

 beginning of my book sufficiently untechnical to be under- 

 stood by the ordinary cultured reader. Having begun in 

 this way, I have tried so to continue throughout the volume, 

 in the hope of interesting a wider audience. This plan has, 

 of course, involved, for the sake of completeness, the cover- 

 ing of much ground which is quite familiar to the general 

 biologist. Hence the inclusion of much that might otherwise 

 be considered as out of place in a book which professes to 

 present fresh aspects or interpretations of problems that are 

 of great current interest. 



I have many to thank for help and advice. Professor 



