250 THE ORIGIN OF SPECIES 



age; and we have only to accelerate partlienogenetic re- 

 production by gradual steps to an earlier and earlier age 

 — Chironomus showing us an almost exactly intermediate 

 stage, viz., that of the pupa — and we can perhaps account 

 for the marvellous case of the Cecidomyia. 



It has already been stated that various parts in the 

 same individual, which are exactly alike during an early 

 embryonic period, become widely different and serve for 

 widely different purposes in the adult state. So again it 

 has been shown that generally the embryos of the most 

 distinct species belonging to the same class are closely 

 similar, but become, when fullj' developed, widely dis- 

 similar. A better proof of this latter fact cannot be 

 given than the statement by Von Baer that "the em- 

 bryos of mammalia, of birds, lizards, and snakes, prob- 

 ably also of chelonia, are in their earliest states exceed- 

 ingly like one another, both as a whole and in the mode 

 of development of their parts; so much so, in fact, that 

 we can often distinguish the embryos only by their size. 

 In my possession are two little embryos in spirit, whose 

 names I have omitted to attach, and at present I am 

 quite unable to say to what class they belong. They 

 may be lizards or small birds, or very young mammalia, 

 so complete is the similarity in the mode of formation of 

 the head and trunk in these animals. The extremities, 

 however, are still absent in these embryos. But even if 

 they had existed in the earliest stage of their develop- 

 ment we should learn nothing, for the feet of lizards and 

 mammals, the wings and feet of birds, no less than the 

 hands and feet of man, all arise from the same funda- 

 mental form." The larvae of most crustaceans, at corre- 

 sponding stages of development, closely resemble each 



