6 EVOLUTION IN COLOR-PATTERN OF THE LADY-BEETLES. 



short of a subspecies when the habitat is made up of many discontinuous 

 areas surrounded by a population of the contrasted forms. 



A variety is a part of the species which differs from the typical members 

 of the species by some hiatus which can be detected by the decreased 

 number or absence of intergrades. When some considerable imperfection 

 of the interf ertility of the apparent variety and the typical species is found 

 or some other cause makes their interbreeding uncommon, the variety 

 becomes a species. Only an arbitrary line can be drawn, and to make an 

 arbitrarily definite one would be infeasible at the present stage of our 

 science. The variety may be equally common throughout the range of 

 the species; it may be much more common in one part; it may be limited 

 to one part of the range; or it may wholly replace the species in one part 

 of the range. If its frequency only gradually changes in one direction, a 

 subspecies is there constituted. If the variety is confined to a particular 

 habitat, we have a habitat- variety. 



An aberration is an individual which is wholly unique, generally patho- 

 logical in origin, and often unsymmetrical. Where the character is sym- 

 metrical and there are no pathological appearances or the variation is of a 

 sort found normally in other species, the specimen is probably one of a 

 rare variety rather than an aberration. 



A mutation is a specimen of a variety which is known to be capable of 

 hereditary transmission, but the ancestors of which are typical individuals 

 of the species. Where this information is lacking the apparent mutation 

 can only be considered to be a rare variety, the origin of which is probably 

 traceable to a progenitor more or less remote. 



An extreme fluctuant is an individual which appears sufficiently dif- 

 ferent to be noticeable, but is, nevertheless, only an extreme case of the 

 ordinary fluctuation, as is shown by the greater frequency of the inter- 

 mediate conditions. 



Form is a term of convenience only, applied to differences the real 

 nature of which is in question. Many forms will later be found to be 

 extreme fluctuants, others may represent points in a fluctuating series 

 easily recognized or described, while still others are doubtless real posi- 

 tions of organic stability, which further study will show to be varieties or 

 intermediate conditions between varieties and continuous variations. 



STRUCTURE OF PIGMENTED AREAS. 



I have limited the work, with a few exceptions, to the color-pattern, be- 

 cause with the other characters we have for the most part stable conditions 

 with only a little fluctuating variation. For the further advantages of 

 specialization I have concentrated my attention upon the pronotum and 

 elytron. I believe, however, that similar results would have resulted were 

 the study extended to include the coloration of the larva and the remaining 

 parts of the imago. The coloration of the pupa, however, is much more 

 subject to modification. 



