Parasitism and Social Reciprocity. 199 



terous insects of which there are thousands of species that deposit 

 their eggs in a caterpillar or other larva, or, as in the case of the sphex, 

 after depositing their eggs in a hole or nest, paralyze another insect or 

 worm and place it in the cell with the egg to be food for the young one 

 as soon as hatched. The young deposited in the bodies of others, as 

 soon as hatched, feed on the abdominal juices of the tissues by which 

 they are surrounded, and often entirely consume their host. 



While this is another phase of parasitism I do not regard it as con- 

 tradictory to the principle announced above, that parasitism comes 

 from the disuse of functions once possessed, and therefore culminates in 

 old age. The disuse of organs in a parasite and their consequent sup- 

 pression or abortion is precisely the same process as that by which the 

 rudimentary organs are formed in all the animals. Wherever we find a 

 rudiment in a vertebrate ( or an invertebrate ) that rudiment is either 

 functional or comes nearer being so in the embr} T o or youthful stage 

 than in the adult. In all cases the suppression of the organ proceeds 

 as the animal grows older. 



The infants of all animals are helpless in the first moments of exis- 

 tence, and must receive food or other attention from some quarter. In 

 the case of mammals the parents take care of their young, and develop 

 a natural supply of food for them. In the case of the lowest marine 

 invertebrates the water, into which they are born, carries food directly 

 to them. If a human mother should feed her baby on cow's milk in- 

 stead of her own, the mother would become, in one sense, a parasite on 

 the cow, and it would correspond with a loss of lacteal function on the 

 mother's part. The child is not parasitic any more by getting its milk 

 from the cow than if it subsisted in the usual way. It is simply an un- 

 developed dependent in either case. The young of the Ichneumon and 

 Sphex are likewise helpless infants, in whose reach food is placed by 

 the precaution of the parents. The adults, in levying on their fellow 

 creatures for the sustenance of their young ones, are like their human 

 brothers, who go to the butcher shop, the dairy, the bee-hive and the 

 hen-house for food to support their young ( and themselves, as well ). 

 The whole of both proceedings only exemplifies the general fact spoken 

 of above, that in animal life such co-operation of tribes as enables a tribe 

 to supersede and get rid of some of its functions by having them per- 

 formed by others, is an essential means of progress. It tears down 

 some old structure which can be got along without, thereby making 

 room for something else. It does not furnish the new function which 

 is now within the possible capacity of the tribe ; the other stimuli of the 

 surrounding environment must do that if it be done at all. In the 

 case of the favored races, as Man and the Ichneumon, the environment 

 finds stimuli to build new functions or intensify the old that are left, 



