4M DEVELOPMENT AND EMBRYOLOGY. 



able that places well adapted for both Mie larval and mature 

 stages, under such new and greatly changed habits of life, 

 would commonly be found unoccupied or ill-occupied by other 

 organisms. In this case the gradual acquirement at an earlier 

 and earlier age of the adult structure would be favored by 

 natural selection ; and all traces of former metamorphoses 

 would finally be lost. 



If, on the other hand, it profited the young of an animal 

 to follow habits of life slightly different from those of the 

 parent-form, and consequently to be constructed on a slightly 

 different plan, or if it profited a larva already different from 

 its parent to change still further, then, on the principle of 

 inheritance at corresponding ages, the young or the larvae 

 might be rendered by natural selection more and more dif- 

 ferent from their parents to any conceivable extent. Differ- 

 ences in the larva might, also, become correlated with 

 successive stages of its development ; so that the larva, in 

 the first stage, might come to differ greatly from the larva 

 in the second stage, as is the case with many animals. The 

 adult might also become fitted for sites or habits, in which 

 organs of locomotion or of the senses, etc., would be useless ; 

 and in this case the metamorphosis would be retrograde. 



From the remarks just made we can see how by changes 

 of structure in the young, in conformity with changed habits 

 of life, together with inheritance at corresponding ages, ani- 

 mals might come tc pass through stages of development, 

 perfectly distinc' from the primordial condition of their adult 

 progenito?s. Most of our best authorities are now convinced 

 that the various larval and pupal stages of insects have thus 

 been acquired through adaptation, and not through inheritance 

 from some ancient form. The curious case of Sitaris — a 

 beetle which passes through certain unusual stages of devel- 

 opment — will illustrate how this might occur. The first 

 larval form is described by M. Fabre, as an active, minute 

 insect, furnished with six legs, two long antennae, and four 

 eyes. These larvae are hatched in the nests of bees ; and 

 when the male bees emerge from their burrows, in the spring, 

 which they do before the females, the larvae spring on them, 

 and afterward crawl on to the females while paired with the 

 males. As soon as the female bee deposits her eggs on the 

 surface of the honey stored in the cells, the larvae of the Sit- 

 aris leap on the eggs and devour them. Afterward they 

 undergo a complete change ; their eyes disappear ; their 

 legs and antenna; become rudimentary, and they feed on 



