PDF - Wallace Online

vii CREATION BY LAW 161cannot be further developed in the same direction. Variationseems to have reached its limits in these birds. But so it hasin nature. The fantail has not only more tail feathers thanany of the three hundred and sixty existing species of pigeons,but more than any of the ten thousand known species of birds.There is, of course, some limit to the number of feathers ofwhich a tail useful for flight can consist, and in the fantail wehave probably reached that limit.Many birds have theoesophagus or the skin of the neck more or less dilatable, butin no known bird is it so dilatable as in the pouter pigeon.Here again the possible limit, compatible with a healthyexistence, has probably been reached. In like manner thodifferences in the size and form of the beak in the variousbreeds of the domestic pigeon is greater than that betweenthe extreme forms of beak in the various genera and subfamiliesof the whole pigeon tribe. From these and facts, manyothers of the same nature, we may fairly infer that if rigidselection were applied to any organ, we could in a comparativelyshort time produce a much greater amount of change thanthat which occurs between species and species in a state ofnature, since the differences which we do produce are oftencomparable with those which exist between distinct genera ordistinct families. The facts adduced by the writer of thearticle referred to, of the definite limits to variability in certaindirections in domesticated animals, are, therefore, no objectionwhatever to the view that all the modifications which exist innature have been produced by the accumulation, by naturalselection, of small and useful variations, since those verymodifications have equally definite and very similar limits.Objection to the Argument from ClassificationTo another of this writer's objections that by ProfessorThomson's calculations the sun can only have existed in asolid state 500,000,000 of years, and that therefore timewould not suffice for the slow process of development of allit isliving organisms hardly necessary to reply, as it cannotbe seriously contended, even if this calculation has claims toapproximate accuracy, that the process of change and developmentmay not have been sufficiently rapid to have occurredwithin that period.His objection to the classification argu-51