74 



external character of structure, colour, and texture is an 

 outward manifestation of some functional character that had 

 its origin in the internal organisation of the animal, and was 

 selected owing to its utility, in some measure at least, to the 

 organism. When a species is well placed with regard to its 

 environment, when the conditions of its existence are such 

 that the species flourishes, a standard or mean is set up, and 

 the species undergoes no change. But the conditions of its 

 environment may be suddenly altered, the drainage of a 

 field, the flooding of hitherto fairly dry land, the introduction 

 of new enemies, the change of wood into field, and similar 

 modifications may bring about suddenly conditions that will 

 lead to the destruction of the species. If it be specialised to 

 some particular marsh food-plant, the drainage of its habitat 

 spells death unless the larvae can accommodate themselves 

 to another food-plant, and this accommodation may bring 

 about a rapid change in the species. The introduction of a 

 new enemy into its haunts may be equally disastrous, unless 

 the individuals become modified in such a way as to combat 

 or deceive the new attacker. The stress of the organic 

 environment due to any peculiar condition or combination 

 of conditions may necessitate a change of location, of habit, 

 of food-plant, &c, with a corresponding change in the 

 organism. But all these contingencies necessitate a change 

 in the organism to meet the new requirements, and the 

 changed organism replaces the old that no longer responds 

 to its environment ; in other words, the old species per se 

 ceases to exist, and the new form takes its place. This mode 

 of development seems to be taking place in many of the pre- 

 dominant groups of the Palaearctic fauna, certain minor 

 divisions of which are now in a state of flux. It is decidedly 

 the case in certain groups of the Coliads, the Melitseids, the 

 Erebiids, the Brenthids, and the Syrichthids. Some forms 

 in each of these groups appear to be simple modifications of 

 existent species, the typical forms having been lost in some 

 localities in the development of the new ones, forms similar 

 to which may even appear as aberrations with the type form 

 in other localities, where the stress of the environment has 

 not necessitated so distinct a change. 



There can be no doubt, apart from the consideration 

 of intra-selection within the organism and its bearing on the 

 final development of certain variational factors, which must 

 in their turn react on the external variational features pro- 

 duced, that natural selection is the prime agent in the 

 moulding of local races, and in time of specific forms. Every 



