686 MAMMALS 



Pure-cone retinae are unknown in mammals outside of the Sciuridae — 

 none occur even in primates, though some of these (e.g. Callithrix 

 jacchus, Cercocebus torquatus) do have many more cones than man. 

 Favorable material of Tupaia has never been studied; and there are still 

 other mammals outside the squirrel and monkey tribes which are re- 

 putedly strongly diurnal, and whose retinae would bear investigation: 

 Ochotona, Dolichotis, Procavia, Cynictis, Suricatd, et al. 



The Early History of the Placentalian Eye — The simplicity of the 

 placentalian visual-cell pattern is striking, when one considers that in 

 the lower mammals each of the standard reptilian-avian cell types is 

 easily recognizable. No placental is known to have double cones, or 

 oil-droplets in its single ones.* Obviously the whole sub-class must have 

 been pulled through some sort of ancestral knot-hole : the 'original' 

 placental mammal must have 'had a way of life which brought about 

 these peculiarities and doomed all of its descendants to exhibit them. 



The whole organization of the monotreme eye is, as we have seen, 

 reptilian. If we think of it as a reptilian eye, its oddities seem logical 

 consequences of a strong nocturnality of long standing. The reversion 

 of the intra-ocular muscles from a striated to an unstriated condition 

 shows that in the first mammals accommodation became unimportant, 

 and it was never necessary for them to close the pupil quickly — presum- 

 ably, because they never exposed themselves to bright light. Accommo- 

 dation is of no value to a strongly nocturnal eye — especially one which, 

 though perhaps relatively large for the animal, is small in absolute 

 dimensions. The discard of the scleral ossicles and the practical discard 

 of the ringwulst of the lens allowed the monotreme eye to become 

 rotund, took the ciliary body out of contact with the lens, and made 

 forever impossible any return to the sauropsidan method of accommo- 

 dation. Though the persistence of the retinal oil-droplets suggests that 

 the early monotremes may have been sufficiently diurnal to have retained 

 the reptilian eye quite unchanged, the nocturnality which eventually 

 supervened accounts for the condition of the modern monotreme organ. 



The marsupial eye, though secondarily arhythmic in capacity in its 

 highest expression (in ground kangaroos) , bears the very same stigmata 



*Little shrinkage spaces at the distal ends of the cone inner segments have been all too 

 often mistaken for oil-droplets — even by such careful workers as Kolmer. Examination of 

 the retina in its fresh condition, and after fixation with osmic acid, will always demonstrate 

 the presence or absence of real oil-droplets. 



