EVOLUTION 



3O3O 



EVOLUTION 



EVOLUTION: THE DEVELOPMENT OF LIFE 



J. Arthur Thomson. LL.D., Regius Prof, of Natural History, Aberdeen 



This important article can well be supplemented by those on Biology ; 



Life ; Sex. See also Cell ; Heredity ; and other articles bearing on 



the subject; also Darwin ; Gallon; biographies of other biologists 



Evolution (Lat. evolvere, to unroll) 

 is a process wherein one kind of 

 living creature gives rise to another 

 kind, which persists alongside of 

 or in place of the original stock. 

 Thus we believe that birds evolved 

 from an ancient reptilian stock, and 

 mankind from a primitive simian 

 lineage, the origins in both cases 

 being extinct. In the case of 

 domestic pigeons derived from the 

 wild rock-dove (Columba livia), or 

 of poultry derived from the jungle- 

 fowl (Gallus bankiva), the origins 

 are still extant. Similarly, wild 

 ancestors of such cultivated plants 

 as cabbages and apple-trees still 

 exist. The evolutionary process is 

 going on among wild plants and 

 animals, e.g. in some evening 

 primroses, or in many birds and 

 butterflies, but it is not readily 

 detected in a lifetime or in the 

 relatively short time since precise 

 biological registration began. 

 What Evolution Means 



While evolution is strictly a 

 slow racial change in living crea- 

 tures, the term is often used much 

 more widely. Evolution should not 

 be confused with development, 

 which is best restricted to a con- 

 tinuous change in one and the 

 same unity, such as a germ, a seed, 

 an organ, an institution, or a solar 

 system. Development agrees with 

 evolution in being a series of 

 changes in a definite direction 

 from one position of equilibrium 

 to another, but differs from it in 

 concerning one and the same in- 

 dividual system from beginning to 

 end, whereas evolution is racial, im- 

 plying a succession of generations 

 and a sifting process. Briefly, 

 development in biology is the 

 individual's coming to be (Onto- 

 geny) ; evolution, in biology, is 

 the genetic history of a race 

 (Phylogeny). Hence we should 

 speak of the development, not of 

 the evolution, of the earth. 



Evolution may be hi the direc- 

 tion of increased complexity and 

 control (differentiation and integra- 

 tion), or in the opposite direction. 

 A tapeworm is the result of a pro- 

 cess of evolution just as surely as is 

 the golden eagle within which it 

 lives. Yet in spite of many in- 

 stances of retrogressive evolution in 

 animate nature, the general trend 

 of the process has been progressive, 

 i.e. towards increased differentia- 

 tion and integration of fuller and 

 freer life. This fact must never be 

 lost sight of in contemplating the 

 history of things as a whole. i 



As applied to living creatures, 

 the evolution theory states the 

 broad idea that the present is the 

 child of the past and the parent of 

 the future. The fauna and flora of 

 to-day, both in themselves and in 

 their myriad inter-relations, are 

 the outcome of an antecedent state 

 of affairs in which animals and 

 plants were on the whole rather 

 simpler. This again originated in 

 organisms and relations simpler 

 still, and so on back through hun- 

 dreds of millions of years, until all 

 clues are lost, and we find our- 

 selves in the mist of life's begin- 

 nings. The evolution theory thus 

 states the view that the manifold 

 intricacy of animate nature has 

 arisen by a natural process of slow 

 organic change, similar to that 

 seen in the history of domestic 

 animals and cultivated plants. 



One point remains to be empha- 

 sised. The statement that living 

 creatures have come to be as they 

 are by evolution, only means 

 that their history has been a 

 natural history, the moves in which 

 have known, or at any rate know- 

 able, causes. To think that any 

 result whatsoever acquires dig- 

 nity, permanency, worth, invulner- 

 ability, or sanctity, because it is 

 the result of evolution, is a misun- 

 derstanding, for the value of sur- 

 vival, as judged by human stand- 

 ards, depends on the conditions 

 under which survival is secured. 

 Evidences of Organic Evolution 



This general evolution theory, 

 or doctrine of descent, cannot be 

 proved like the law of gravitation. 

 It is the only scientific way of 

 answering the question : How 

 has the present-day system of 

 animate nature come into being ? 

 But while all the facts of zoology 

 and botany serve as evidences of 

 evolution, four main lines of argu- 

 ment have been followed by 

 Darwin and others. 



The first is mainly ana- 

 tomical. Many facts in regard to 

 structure corroborate the evolu- 

 tionist interpretation, and seem to 

 naturalists to admit of no other. 

 Thus, the fore -limb of a frog, the 

 paddle of a turtle, the wing of a 

 bird, the fore -leg of a horse, the 

 flipper of a whale, the wing of a 

 bat, the arm of a man, exhibit in 

 diverse guise the same essential 

 parts, twisted into manifold forms 

 for different uses, but always of the 

 same fundamental type. There is 

 essential similarity in the import- 

 ant bones, and considerable re- 



semblance in the musculature, in- 

 nervation, and blood-supply. All 

 these fore-limbs are homologous 

 with one another, i.e. they agree 

 in fundamental structure and de- 

 velopment. It is difficult to 

 understand this adherence to 

 type except on the theory of the 

 actual flesh-and-blood relationship 

 of backboned animals. Many ves- 

 tigial organs in animals, especially 

 the higher animals, remain very 

 slightly developed and are of no 

 use; comparable, as Darwin said, to 

 unpronounced letters in words, the 

 o in leopard, or the b hi doubt. 

 Man has a minute useless third 

 eyelid and a hint of muscles for 

 moving the trumpet of the ear. The 

 only rational interpretation of such 

 structures is the evolutionist one, 

 that they are dwindling relics of 

 structures well developed and of 

 some functional importance in 

 ancestral forms. 



The Physiological Argument 



The second line of argument 

 may be called physiological. When 

 the blood of a horse is transfused 

 into an ass, or that of a hare into a 

 rabbit, there is harmonious blend- 

 ing. But when human blood is 

 transfused into a horse or rabbit 

 there is great disturbance, marked, 

 for instance, by destruction of red 

 blood corpuscles. The harmonious 

 mingling is evidence of near blood- 

 relationship, the destructive reac- 

 tion proves the reverse. By modi- 

 fication of this experiment it is 

 possible to gauge the degree of 

 relationship between man and 

 the various groups of apes and 

 monkeys. Along with this physio- 

 logical argument may be taken the 

 abundant evidence of the varia- 

 bility of living creatures. In a 

 short time man has established 

 over 200 breeds of domestic 

 pigeons, which seemallto have been 

 derived from the blue rock-dove. 



Another line of argument 

 is historical or palaeontological. 

 From the rock record we have ac- 

 cumulated a great mass of material 

 in regard to the successive appear- 

 ance of horse-types, elephant- 

 types, crocodile-types, and so forth, 

 all reading like a lineage or pedi- 

 gree. Moreover, there are many 

 connecting links now extinct, 

 such as Archaeopteryx (see Birds), 

 which, though an indubitable bird, 

 had several well-marked reptilian 

 features, e.g. teeth in both jaws, a 

 lizard -like tail, and claws on the 

 three digits of the hand. Again, 

 there is the big fact that in the 

 rock record amphibians appear 

 after fishes, reptiles after am- 

 phibians, birds and mammals after 

 reptiles ; as age succeeded age, 

 nobler and nobler forms of life 

 emerged. 



