284 ADAPTATIONS TO SPACE AND MOTION 



and insectivores (see Fig. 60, p. 135), they have had to get along with 

 whatever portions of the beautiful sauropsidan mechanism those antique 

 nocturnal mammals happened to retain. 



That was not much, for aside from the ciliary muscle itself — and this 

 has retrograded to the unstriated type — not one of the sauropsidan 

 adjuncts to vigorous accommodation remains in any mammal. Though a 

 slight circumcorneal sulcus (marked in apes and man) may be present, 

 it is not an indentation of the sclera itself and is never supported by 

 scleral ossicles; nor is there ever an annular pad on the lens. The mono- 

 tremes have a vestige of the pad and have kept the scleral cartilage, thus 

 presenting a tunica fibrosa which is matched in the sauropsida only in 

 the crocodiles — likewise nocturnal and primitive within their class. In no 

 mammal are the ciliary processes joined to the lens capsule, and in only 

 a few are they ever even in light contact with it during accommodation. 

 This simplification of the mammalian eye, giving it an essentially 

 amphibioid make-up, has led one prominent phyleticist (Franz) to 

 suggest that the placental mammals were derived from forms intermedi- 

 ate between the amphibians and the reptiles, with only the monotremes 

 and marsupials (the former having scleral cartilage, and both groups 

 showing double cones and oil-droplets in their retinae) tracing back to 

 fully differentiated reptiles. 



The comparative anatomy and palaeontology of the occipital condyles 

 would seem to make such a diphyletic origin of the mammals quite im- 

 possible. The placental mammals lack so many of the sauropsidan ocular 

 structures, not because their ancestors never had them, but because the 

 small-eyed sub-insectivores were so strictly nocturnal that they discarded 

 these daytime features as so much excess baggage. The oldest known 

 mammals averaged less than rat-sized and are indicated, by their den- 

 tition, to have been insectivorous and granivorous. Wherever among the 

 placental mammals very small size has reappeared, even the inferior 

 mammalian mechanism has failed to evolve, or has been allowed to 

 disappear. 



The most important result of the wholesale discardments of reptilian 

 ocular structures in the mammals has been to take the ciliary body out 

 of intimate contact with the lens — especially far out, in the simians and 

 the echidnas, despite the breadth of their lenses. Those semi-diurnal and 

 diurnal mammals which have rebuilt an effective accommodation have 

 consequently (like the snakes) been under the necessity of developing 

 a brand-new method. 



