NEUTER INSECTS. 219 



upon which natural selection has full opportunity to produce its effects. 

 Originally it may have simply rested for a time in the direct line of de- 

 velopment, on account of finding abundant food. But, as food condi- 

 tions changed, new enemies attacked it, or old foes adopted new modes 

 of assault, one of two things was necessary for its survival. It must 

 either lose this resting-stage and develop continuously, or it must be- 

 come adapted to the new conditions. This rendered necessary changes 

 in instinct and in structure. Where the resting-stage, as in the cater- 

 pillar, occupied a very large percentage of the total life-duration, and 

 where the process of adaptation had millions of years for its comple- 

 tion, it is not surprising that structural features often very divergent 

 from the typical form were assumed. 



There is little or no reason to doubt that all the peculiarities of lar- 

 val form are due to the two causes here specified : 1. A temporary 

 check to development at some ancestral stage of the animal's unfold- 

 ment. 2. An adaptive modification of structure and habit to meet 

 varying conditions in the environment of this stage of development. 



Yet in every such case we meet with a difiiculty of the same char- 

 acter as that existing in the case of neuter ants. These secondary 

 adaptations are out of the direct line of the animal's development, and 

 it is a question how they can be hereditarily transmitted. The law of 

 phylogenetic development enables us to understand the appearance 

 of certain embryonal peculiarities of structure which do not exist in the 

 mature form. If development is forced to follow its original line, such 

 ancestral features must necessarily a2)pear, though if the development 

 is very rapid only hints of them are perceptible ; or they may become 

 utterly obliterated, so far as our powers of obsei'vation can decide. 

 Yet such a principle can not apply to secondary structural features, 

 produced in larval adaptation. The latter are in no sense in the direct 

 ancestral line of development, and it is somewhat remarkable that they 

 are so faithfully reproduced, only to be thrown aside again as the 

 animal resumes its temporarily checked development. 



It is very evident, from the facts here cited, that the phylogenetic 

 line is subject to disturbing influences. There is no special reason, in 

 the nature of things, why a developing animal should repeat every 

 stage of its ancestral growth. If never disturbed in its development 

 it would naturally do so, since its original evolution from primeval 

 matter lay in that line, and there has been no force since brought to 

 bear upon it to make it deviate. But where any subsequent force 

 causes deviation, that deviation must become persistent. There can 

 be no possible return to the exact ancestral course. Many such devia- 

 tions have occurred. Some of them are only apparently such, arising 

 from rapidity of development, and the slurring over of intermediate 

 steps in the line of growth. But many of thom are results of subse- 

 quent adaptation. Such is the case with many of the peculiarities seen 

 in the unfoldment of the mammalian embryo. It has deviated from 



