THEORIES AND MECHANISM OF EVOLUTION 



659 



portunities by providing a new environ- 

 ment; the body of a vertebrate as a home 

 for a blood parasite is an ilhistration. Thus 

 new environments are constantly being cre- 

 ated ( and others destroyed ) which open up 

 more and more opportunities for a greater 

 variety of living things. This has produced 

 an untold number of kinds of animals,, of 

 which probably something over a million 

 are alive today. 



Evolution constantly operates with ma- 

 terials available and does the best it can 

 with them to satisfy a need, frequently pro- 

 ducing a structure that may not be perfect 

 but serves the purpose more or less satis- 

 factorily. Perhaps there is no structure in 

 the body of any animal that performs its 

 "intended" function to a perfect degree. 

 Most of them do a tolerable job, some bet- 

 ter than others, but nowhere do they func- 

 tion perfectly. Certainly there must be 

 some design that is perfect. Why, then, are 

 not all of the organs of animals perfect, 

 or more broadly, why is the animal itself 

 more or less imperfect in every detail? 

 There is constant selection toward perfec- 

 tion, but complete and absolute fitness to 

 the environment is never quite attained, 

 as can be seen by a careful examination of 

 any animal, including man himself. 



To illustrate this point, any one of hun- 

 dreds of examples in any animal or group 

 of animals might be chosen. The matter of 

 photoreception, which was reviewed pre- 

 viously in the various animal groups, is a 

 good illustration of how all groups of ani- 

 mals have devised some method of receiv- 

 ing light rays. This is an important adjunct 

 to orientation in the environment and, ac- 

 cordingly, every animal can make good use 

 of such an organ. Among all of the photo- 

 receptors there is none that we might call 

 a perfect photoreceptor. The primate eye is 

 the best and we are all aware of is imper- 

 fections. We have been able to build a bet- 

 ter one in the laboratory — one that will do 

 things the human eye cannot approach. 

 Furthermore, our eyes seem to do a rather 



good job during adolescence and early ma- 

 turity but fail in the later years, an imper- 

 fection that might be due to the fact that 

 in their evolution they were not "intended" 

 to last into old age, or perhaps the body 

 itself was not "intended" to survive past its 

 period of optimal activity. That is, evolu- 

 tion has tended toward the production of a 

 satisfactory animal body until after the 

 young are reared, after which it becomes 

 more imperfect, eventually causing the 

 death of the entire organism due to the fail- 

 ure of its separate parts. 



Animals have experimented with photo- 

 receptors and seem to have tried about 

 every possible way of receiving light, from 

 very simple stigmas in the Protozoa to the 

 complex molluscan and vertebrate eyes. All 

 gradations are found from those that merely 

 record light and darkness to those that form 

 crude images and finally to those that pro- 

 duce clear, distinct images. We need only 

 recall the crude patches of pigmented cells 

 in planaria ( Fig. 9-6) to see a photoreceptor 

 that is very primitive yet sufficient for the 

 species to survive. A better eye would be of 

 no value because the animal does not pos- 

 sess a nervous system adequate to record 

 such impressions if they were received. 

 Hence, the eye has paralleled the develop- 

 ment of other organs of the body, and has 

 usually been as good as needed to maintain 

 the species. We might argue that man could 

 conveniently use an eye with the speed of 

 perception that is possessed by a photo- 

 electric cell, but our brains could not re- 

 ceive and interpret the impressions that 

 would come through such eyes. 



From the planarian eye we see steady 

 improvement concomitant with the devel- 

 opment of organ systems until finally 

 among the mollusks, artlu'opods, and ver- 

 tebrates we see the best eyes of all. It is 

 interesting to note that each of these types 

 has faced the same problem but has solved 

 it differently. The arthropod eye consists of 

 many tiny units aggregated into a com- 

 pound structure which forms many images. 



