The Consequences of Evolution - 553 



EVOLUTIONARY BASIS OF CLASSIFICATION 



According to their similarities and differ- 

 ences, organisms have been classified into 

 groups and subgroups — that is, into the vari- 

 ous phyla, classes, orders, and so forth — of 

 the plant and animal kingdoms (Appendix 

 I). This classification is not an arbitrary sys- 

 tem, as is witnessed by the fact that all or- 

 ganisms in a given group, however large this 

 group may be, resemble each other not 

 merely as to one characteristic, but as to 

 many. Originally the classification was laid 

 out on the basis of the gross structures mu- 

 tually possessed by members of each group, 

 but subsequent study has usually revealed 

 many other resemblances — as to develop- 

 mental, biochemical, and physiological fea- 

 tures — which were entirely unknown to the 

 biologists who first devised the classification. 

 Such facts very clearly indicate that there 

 are real, natural relationships underlying the 

 classification system. In fact, the main intent 

 of classification is to group organisms accord- 

 ing to their genetic relationships with other 

 organisms. On the average, those organisms 

 most closely related by descent will mutually 

 possess the greatest number of similar or 

 identical genes, and organisms possessing the 

 greatest common fund of similar genes will 

 resemble each other most closely as to their 

 phenotypic characteristics. Of course there 

 are exceptions to these general rules: iden- 

 tical genes may arise independently even in 

 species of fairly distant relationship, and, on 

 occasion, the phenotypic effects of one set of 

 genes may simulate quite closely the effects 

 of a different set of genes. Therefore, re- 

 semblance in one or a few characteristics is 

 not a safe criterion of genetic relationship. 

 But it is not probable that such coincidences 

 should occur frequently, and the more char- 

 acteristics a group of organisms possess in 

 common, the greater is the certainty that 

 these resemblances are due to common genes 

 derived from a common ancestry. And when- 

 ever it is possible to test this question by 

 breeding experiments, organisms that appear 



to be most closely related on the basis of 

 their phenotypic features are in fact gen- 

 erally found to possess the greatest number 

 of common genes. Actually the modern sys- 

 tem of classification considerably antedates 

 the methods of modern genetics, but the 

 criteria that were used in establishing the 

 classification of organisms have proved, on 

 the whole, to be quite valid. 



It is possible to subdivide existing species 

 into well-defined groups, only because a large 

 proportion of intermediate organisms have 

 become extinct. If every species and sub- 

 species that ever lived were still alive today, 

 there would be no definable groups, but a 

 continuous series of intermediate forms con- 

 necting every type of organism with every 

 other type by insensibly small degrees of 

 difference. Classification depends in a large 

 measure upon the accidents of extinction, 

 and thus we do not find a neat symmetrical 

 scheme, but rather a confusing system of 

 large, small, well-defined, ill-defined, re- 

 motely different, and closely similar species, 

 genera, families, orders, classes, and phyla, 

 making up both the plant and animal king- 

 doms. Existing species are frequently and 

 aptly compared to the surviving twigs of a 

 buried tree, of which the main limbs and 

 branches have decayed and disappeared. 

 Classification must seek to assign each twig 

 to the proper branch and subbranch from 

 which it initially originated. In many cases 

 this task is simplified by the uncovering of a 

 few more or less imperfect remnants (fossils) 

 of the former connections; but under the 

 circumstances it is not surprising that there 

 are many uncertainties and differences of 

 opinion. In some cases there are so many in- 

 termediate forms between two different 

 groups that the line of separation must neces- 

 sarily be altogether arbitrary — as is the case 

 of the line that separates the animal and 

 plant kingdoms. In other cases the opposite 

 difficulty arises: so much extinction has oc- 

 curred that certain small groups of species 

 are left, isolated with no obvious relations 

 to any other larger group — as in the case of 



