MUTUAL AFFINITIES OF ORGANIC BEINGS 403 



extent. Differences 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 be- 

 come fitted for sites or habits, in which organs of locomotion or of 

 the senses, etc., would be useless; and in this case the metamor- 

 phosis 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, animals might 

 come to pass through stages of development, perfectly distinct 

 from the primordial condition of their adult progenitors. Most of 

 our best authorities are now convinced that the various larval 

 and pupal stages of insects have thus been acquired through adap- 

 tation, and not through inheritance from some ancient form. The 

 curious case of Sitaris — a beetle which passes through certain un- 

 usual stages of development — ^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 Sitaris leap on the eggs and devour them. After- 

 ward they undergo a complete change ; their eyes disappear ; their 

 legs and antennae become rudimentary, and they feed on honey; 

 so that they now more closely resemble the ordinary larvae of in- 

 sects; ultimately they undergo a further transformation, and 

 finally emerge as the perfect beetle. Now, if an insect, undergoing 

 transformations like those of the Sitaris, were to become the pro- 

 genitor of a whole new class of insects, the course of development 

 of the new class would be widely different from that of our exist- 

 ing insects; and the first larval stage certainly would not repre- 

 sent the former condition of any adult and ancient form. 



On the other hand, it is highly probable that with many animals 

 the embryonic or larval stages show us, more or less completely, 

 the condition of the progenitor of the whole group in its adult state. 

 In the great class of the Crustacea, forms wonderfully distinct from 

 each other, namely, suctorial parasites, cirripedes, entomostraca, 

 and even the malacostraca, appear at first as larvae under the nau- 

 pliusform ; and as these larvae live and feed in the open sea, and are 

 not adapted for any peculiar habits of life, and from other rea- 



