PARASITIC WEAVERBIRDS 35 



ing habits within the group were such that at more than one point 

 occurred the necessary combination of loss of nest building and reduc- 

 tion of broodiness. Since loss of nest building, as expressed by- 

 habitual use of adopted, deserted, or usurped nests built by other 

 birds, frequently occurs among ancestors of both parasitic lines, 

 probably this state of affairs was the underlying one, and the decrease 

 or total loss of broodiness was superimposed on it. I have noted 

 evidence suggestive of this condition in individual cases of several 

 species that still ordinarily take care of their eggs. P'ortunately, w^e 

 can complement these data with others pertaining to the occasional 

 occurrence of bits of atavistic selfbreeding behavior in the most 

 primitive of the parasitic viduines, the combassous, and also in two 

 of the more advanced viduine species, the pin-tailed and paradise 

 widow birds, even if only under the artificial conditions of aviary 

 living. The data pertaining to the ethological variations in the nature 

 of the ancestral stock of selfbreeding weavers show that there was a 

 fertile ground for the development of brood parasitism within the 

 group. The atavistic glimpses afforded by the most primitive of the 

 parasites reveals the past from which they developed. 



An essentially comparable situation in parasitic invertebrates w^as 

 reported by Baer (1951, p. 211), who found that many organisms 

 give evidence of having been "predisposed to becoming parasitic, 

 and that they w^ere preadjusted to a different mode of life that has 

 enabled them to establish themselves successfully in a new biological 

 niche." 



The diverse antecedent ethological variations that we have reviewed 

 constitute essentially some grade or degree of preadaptation toward 

 brood parasitism. In this connection I note that in his study of 

 recent views of evolutionary processes, Carter (1951, p. 275) summed 

 up the matter of preadaptation as follows: "In its most general 

 sense preadaptation implies merely the possession of characters 

 suitable for use in some situation, and the possession of these charac- 

 ters before the situation arises, before there is any opportunity for the 

 use to which they are preadapted. In all adaptation the animal 

 must to some extent be preadapted in this sense; it must always possess 

 characters that can be modified to give the adapted character." 



In his general discussion of avian brood parasitism, Miller (1946, 

 p. 246) came to generally similar conclusions. "Resort to parasitism 

 under several sets of circumstances has not presented great evolu- 

 tionary difficulties, although its appearance seemingly has depended 

 on the presence of some essential preadaptations that chanced to be 

 available. Having once taken the decisive step and become depend- 

 ent, the parasitic species may have added refinements, such as rapid 

 delivery of eggs, killing of competing young, and mimicry of egg 



