INHERITANCE OF ACQUIRED CHARACTERS 



207 



B 



from the northeast and the northwest of the United States are 

 darker in color than those from the interior, and again that red 

 shades are more common in the arid southwest. Similar effects 

 have been recently shown by a study of species of wasps. Modi- 

 fications of this type may be produced at will by subjecting 

 the larvae and pupae of certain in- 

 sects to artificial heat and cold. 

 The butterflies of the glacial regions 

 and those developed in the ice chest 

 have a pale coloration, and a warm 

 environment deepens the pigment. 

 The woodpeckers and other birds of 

 the rainy forests, northwest and 

 northeast, have always darker and 

 more sooty plumage than those birds 

 of the same type found in more 

 sunny regions. 



A typical case is found in the 

 various species of sticklebacks (Fig. 

 119) constituting the genera Gaster- 

 osteus and Pygosteus of the Northern 

 Temperate Zone. In both genera, 

 the marine species are armed for the 

 whole length of the body by a series 

 of about tw r enty to thirty vertically 

 oblong enameled bony plates. 



In brackish waters in Europe, 

 America, and Asia alike, the stickle- 

 backs in all the various species are 

 only partially mailed, having vari- 

 ously from three to fifteen bony 

 plates, these smaller than in the 

 marine forms and covering only the 



anterior part of the body. In these fishes also, the spines of 

 the fins are less developed than in the marine forms. In 

 strictly fresh waters, sticklebacks of various types are found 

 entirely destitute of bony plates. These unarmed fishes have 

 been regarded as distinct species and as distinct subspecies. 

 At present they are usually simply regarded as variant :< forms," 

 to which distinctive scientific names need not be applied. It 

 has not been proved, but it is probably a fact, that the differ- 



FIG. 118. Specimen of Cera- 

 tiurn, collected (A) out of the 

 Guinea Coast stream and (B) 

 out of the South Equatorial 

 stream; note the marked dif- 

 ference in development of the 

 spines. (After Weismann and 

 Chun.) 



