i6 4 CONVERGENCE IN MINUTE STRUCTURES 



lamprey, Dohrn, committed to the Annelid Theory 

 of vertebrate descent, should have concluded 

 that the endostyle was derived by descent with 

 simplification from the more elaborate organ in 

 Ammocoetes, in which opinion he has been sup- 

 ported by Gaskell. Regressive convergence by 

 reduction and loss of parts is indeed a very 

 common phenomenon, expressive of the level- 

 ling effects of degeneration, e.g., the loss of 

 limbs in snakes, some lizards, some batrachians, 

 and some fishes ; compare also the effects of 

 sedentary and parasitic habits. In less obvious 

 cases this principle has to be applied with the 

 utmost circumspection. 



For Gaskell's views on the homology which 

 he supposes to exist between the thyroid gland 

 of Ammocoetes and the uterus of the scorpion, 

 the reader should consult the work quoted above. 

 For my present purpose the following passage 

 will suffice {op. cit., p. 205) : — "The resemblance 

 between the structure of the thyroid of Ammo- 

 coetes and the uterus of the scorpion is most 

 striking, except in two respects, viz., the nature 

 of the lining of the non-glandular part of the 

 cavity — in the one case ciliated, in the other 

 chitinous — and the place of exit of the cavity, 

 the thyroid of Ammocoetes opening into the 

 respiratory chamber, while the uterus of Scorpio 

 opens direct to the exterior." It may be added 

 by way of explanation that the supposed homology 



