538 CLARENCE E. McCLUNG 



nished by the facts disclosed in the study of Hesperotettix and 

 Mermiria. For convenience these facts may be taken up under 

 a number of different headings. 



1. Chromosome numbers in the Acrididae 



'It is claimed by those who criticise the so-called chromosome 

 theory of heredity that the maintenance of the specific number 

 of chromosomes is required and that variations in number are 

 direct disproof of the theory. There is an element of truth in 

 this argument which has always been granted by students of 

 cellular phenomena, but stated baldly and without reservation it 

 may lead to entirely erroneous conclusions — ^indeed has done so 

 in conspicuous instances. The fallacy in the argument lies in 

 the circumstance that a primary organization may be maintained 

 while having superimposed upon it secondary modifications. 

 Instances of this are very common in organic structures. It is no 

 argument against the reality of the pentadactyl type of limb that 

 duplications or combinations or reductions occur in certain groups 

 or individuals. Polydactyl or syndactyl individuals reproducing 

 their modifications of the primitive type are not regarded as 

 illustrations of the absence of a fundamental organization of 

 limb bones. The diplopod condition is rightly considered as 

 secondary to the usual arthropod arrangement — not a disproof of 

 its existence. While it is true that these instances of organiza- 

 tion are of a different order from those found in cellular structures 

 they are true examples of meristic variation in individuals in 

 the presence of a persistent tj^pe. All that is claimed by those 

 who believe in the hereditary significance of nuclear structures is 

 that the chromosomes are, in themselves, indications of structural 

 organization in the materials of which they are composed. Pro- 

 vided the full complex of elements be preserved, the essentials 

 of idioplasmic control of development exist, even in the pres- 

 ence of combinations or duplications. From this point of view, 

 in the event of apparent variation in chromosome numbers in 

 an individual or group, it is essential to discover whether any 

 of the normal chromatic units are lost or others of different 



