64 AGE, GROWTH, AND DEATH 



chosen a section of the retina (Fig. 21). One can see 

 near the top of the figure the peculiar cylindrical and 

 tapering projections (rods and cones) which are 

 characteristic of a retina, projections which are of 

 especial interest because they represent the apparatus 

 by which the rays of light are transformed into an 

 actual sensory perception. After this has been ac- 

 complished, the perception is transmitted into the in- 

 terior substance of the retina, and by the complication 

 of the figure you may judge a little of the complica- 

 tion of the arrangements by which the transmission 

 through this sensory organ is achieved, until the 

 perception is given off to a nerve fibre and carried to 

 the brain. There is not time to analyse all I might 

 present to you of our present knowledge concerning 

 the structure of the retina. But it will, I think, suf- 

 fice for purposes of illustration to call your attention 

 to the complicated appearance of the section as a 

 whole and to assure you that nothing of the sort 

 exists in the early stage of the embryo. 



To recapitulate, then, what we have learned from 

 the consideration of these pictures, we may say that 

 in place of uniformity we now have diversity. It 

 should be added, to make the story complete, that 

 the establishment of this diversity has been gradually 

 brought about, and that what we call development is 

 in reality nothing more than the making of diversity 

 out of uniformity. It is a process of differentiation. 

 Differentiation is indeed the fundamental phenome- 

 non of life ; it is the central problem of all biological 



