CHAPTER XV 

 THE EYES OF MAMMALS 



The portrait of geobge lindsay johnson (1853-1943) (Fig. 535) seems 

 to be a suitable introduction to this chapter on the mammaUan eye. He was 

 one of those extraordinary people whose life was full of interest and odd 

 happenings. Born in England, in Manchester, he received much of his early 

 education in Germany and for that purpose was in Strasbovu-g when it was taken 

 by the Germans in 1870. Thereafter he completed his medical studies and 

 ophthahnic training in London, leaving in 1911 for South Africa where he died 

 at the age of 90. In London he spent most of his spare time in the Zoological 

 Gardens where he stvidied intensively the comparative anatomy of the eye, 

 making contributions to the Royal Society on the eyes of Reptiles, Amphibians 

 and Mammals. This interest he maintained to the end. So enthusiastic, indeed, 

 was he that at an advanced age, determined to observe the fundus of the whale 

 in life, he joined a whaling expedition, had a special crane built on the deck of 

 the ship and had himself lowered over the back of the animal so that he could 

 sketch its fundus. His Pocket Atlas of the Fundus Oculi is well knowTi ; and his 

 extraordinary versatility is exemplified in the many optical instruments which 

 he devised as well as his pioneer work in colour photographj% a subject in which 

 he maintained an interest to the end of his life. 



1VIAM1VIAI.IA, the highest class of the Vertebrates, have evolved from primitive 

 Reptiles on diverging lines from the Birds^; both classes show high adaptations, 

 and if the Birds possess the air. Mammals possess the earth although a few have 

 taken to the air and more to the trees, while others have become amphibious or 

 aquatic. The Mammals, however, have two distinctive peculiarities — the 

 elaboration of the brain and the intimate organic connection between mother 

 and offspring. They possess in common several characteristic features — a 

 covering of hair, a diaphragm and a foi.u'-chambered heart, three auditory 

 ossicles and a three-chambered ear, a single jaw-bone, and — a circumstance 

 peculiar to Mammals — the young are nourished by milk secreted from the female 

 mammary gland. The eyes are not so fully develoj^ed as those of Birds, but 

 their comparative anatomical simplicity is more than compensated functionally 

 by the efficiency of the central nervous organization of vision. 



From the ocular point of view — and from practically every other point 

 of view — the extant members of the class are divided into three subclasses, which, 

 it should be remembered, are not linearly derived the one from the other : 



( 1 ) The PEOTOTHERiA or MONOTREMES which are oviparous, the yoi.mg being 

 hatched from eggs outside the body. 



(2) The METATHERiA or MARSUPiAXS, in which the young are born in an 

 immature state and are (generally but not invariably) nourished and protected 

 for some time in an external pouch (or marsupium). 



(3) The EUTHERiA or placentaxs, in which the young are nourished within 

 the uterus through the placenta until development is far advanced. It is among 

 the Placentals that cerebral advancement begins to be marked. 



1 p. 234. 



429 



