EXTENSIONS OF DARWINISM 263 



ments might arise, usually, no doubt, directed by utility to 

 the species. Thus the glorious train of the peacock might 

 have begun in mere density of plumage covering a vital 

 part and one specially subject to attack by birds or beasts 

 of prey, and, once started, these plumes would continue to 

 increase in number and size, as being an outlet for vital 

 energy, till at last they became so enormously lengthened 

 as to become dangerous by their weight being a check to 

 speed in running or agility in taking flight. This is already 

 the case with the peacock, which has some difficulty in 

 rising from the ground and flies very heavily. Its enemies 

 in India are tigers and all the larger members of the cat- 

 tribe, and when any of these approach its feeding-grounds 

 it takes alarm and at once flies up to the lower branches of 

 large trees. In the Argus -pheasant it is the secondary 

 wing-feathers that are exceedingly long and broad, so as to 

 be almost as much a hindrance to strong or rapid flight as 

 is the train of the peacock ; and in both birds these orna- 

 mental plumes have evidently reached the utmost dimensions 

 compatible with the safety of the species. 



There can also be little doubt that in many of the birds- 

 of-paradise and of the humming-birds, in the enormous crest 

 of the umbrella-bird, in the huge beaks of the hornbills and 

 the toucans, in the lengthy neck and legs of the flamingos 

 and the herons, these various ornamental or useful append- 

 ages have reached or even overpassed the maximum of 

 utility. In another class of animals we have the same 

 phenomenon. The expansion of the wings in butterflies 

 and moths reaches a maximum in several distinct families — 

 the Papilionidse, the Morphidae,the Bombyces, and theNoctuae, 

 in all of which it is sometimes from nine to ten inches. 

 Here, again, we seem to find a tendency to development 

 in size, which has gone on from age to age, till limits have 

 been reached to exceed which threatens the existence of the 

 species. 



The progressive development of many groups of animals 

 affords curious illustrations of this continuous increase in 

 bulk, or in the size of particular organs, till they have 

 actually overpassed the line of permanent safety, and under 

 the first adverse conditions have led to extinction. Both 



