498 THE CHANGING WORLD 



An outstanding example of common origin that is frequently cited 

 is the case of homologous bones in the wing of a bird, the leg of a 

 quadruped, the flipper of a whale, and the arm of man, which con- 

 form to a common plan but develop into diverse structures for differ- 

 ent uses. 



One of the blood ties that suggests the cousinship of all verte- 

 brates is the fact that they are limited to two pairs of lateral append- 

 ages, although some of them have lost one or both of these pairs. 

 A learned doctor's thesis in biology that could satisfactorily explain 

 the presence of wings in addition to arms on the shoulders of the 

 angels which Raphael painted on the ceiling of the Sistine Chapel 

 in Rome would be as famous as the frescoes themselves. The only 

 possible conclusion acceptable to the comparative anatomist would be 

 that angels and men are entirely unrelated, which may be true enough. 

 When representatives of animal groups are passed in review in 

 the mind's eye, all sorts of different structures, such as skeleton, 

 kidneys, teeth, sense organs, brains, and respiratory devices, fall into 

 line as being made up of a continuous series, explainable upon the 

 supposition that they have evolved one from another during the 

 long course of geologic time, but that otherwise are unintelligible. 

 It is quite impossible for a comparative anatomist, grounded in the 

 knowledge of many details, not to be convinced that the leg of a 

 horse is one end of a series of structural modifications that began 

 with the fin of a fish. Or that the plan represented in the life cycle 

 of the flowering plants is not the outcome of the alternation of gamet- 

 ophytes and sporophytes so apparent in the mosses and ferns. All 

 the necessary connecting links are there, which would be senseless 

 indeed if they did not fall into line to spell continuity. Innumerable 

 examples of this sort are familiar to the biologist. 



As has been pointed out by certain doubting Thomases, the fact 

 that things may be arranged in a continuous series does not neces- 

 sarily mean genetic relationship. Weapons of human defense, for 

 example, may be traced from their earliest beginnings in the form 

 of stones and clubs, up through spears, bows and arrows, and fire- 

 arms of increasing efficiency, to the deadly machine gun, yet no one 

 would say that this is genetic derivation of one kind from another, 

 because weapons of defense do not reproduce their kind as do living 

 things. The U. S. Patent Office is well aware that the same idea 

 often turns up from widely different sources having no possible 

 immediate connection. 



