EVOLUTION, NAMES, AND CLASSIFICATION 71 



Let us return to the butterfly fish population on the isolated reef which was 

 previously mentioned. Suppose that these fish were at one time plainly colored, 

 small yellow fish without a vertical stripe through the large black eye. Suppose 

 that one individual was the result of a mutation which caused a narrow stripe 

 to appear through its eye. This stripe would be an advantage in camouflaging 

 the otherwise very noticeable eye (Chapter 9), and the gene controlling 

 the stripe could be said to have a plus selective value. It would aid this fish to 

 avoid its predators a little better, give it a little more time to eat, and allow it a 

 slightly longer life in which it could produce more off^spring, some or most of 

 which would also have this camouflaging eye stripe. Eventually, stripeless in- 

 dividuals would be rare or lacking. More mutations might also occur to strengthen 

 this trend toward a stripe through the eye in butterfly fishes. Such a trend is, in 

 fact, very evident in these fishes. 



But evolution cannot be concerned merely with the change within one 

 population. Somehow there must arise. a situation whereby two types are pro- 

 duced from one, a splitting by which new species, families, and higher categories 

 may arise out of the old. This can only occur if somehow the population is 

 broken into two parts which cannot interbreed with one another. Suppose one 

 segment of the butterfly fish population was swept away or wandered away 

 to another distant reef. There would now be two populations on two different 

 reefs. These populations would have slightly different mutations, and the reefs 

 would represent slightly different environments. The results of the evolution 

 in these populations would thus be different, and, given time, two new species 

 would evolve which, even though rejoined, might have become so different that 

 breeding would not occur between them. With expansion of this process of 

 splitting, new families, orders, and higher groups may come into being. It is 

 not too difficult to imagine how, by this process, certain fishes in shallow waters 

 became amphibians adapted to spend part of their lives on land by selection 

 in favor of supporting limbs, how certain amphibians became reptiles adapted 

 to spend all their lives on land by selection in favor of an impervious dry skin, 

 how certain reptiles became mammals adapted for active life by selection in 

 favor of warm-bloodedness, and how certain apes became men adapted for con- 

 templative life by selection in favor of high-mindedness. In each case, some of 

 the descendants of the groups which gave rise to higher categories remained 

 behind, so we see that evolution is a process that creates new forms and builds 

 on itself. The nineteenth-century biologist Thomas Huxley likened evolution 

 in filling the available living space on earth to the filling of a barrel. First, the 

 barrel may be filled with apples. Then pebbles may be put into the spaces 

 between the apples. Sand fills the spaces between the pebbles and finally water 

 may be poured in to fill the barrel. Simpson (1949) adds that the barrel itself 

 expands as the new forms of life create new ways of life bv their verv presence. 

 For instance, without birds, there would be no bird lice, without insects no 

 insect-pollinated flowers, and without whales, no whale barnacles. 



Evolution is studied from three aspects chiefly. First, the history of evolution 

 is given in fossil form— paleontology. Second, the mechanics of evolution are 

 revealed by genetics. Third, the all-important environmental effect is examined 

 by ecology, commonly known under the name of natural history. We have 

 briefly explained the first two. This book mainly concerns ecology, and three 



