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Show 1903.] o x TFJE TRANSPOSITION OF MAMMALIAN TESTES. 319 4. On the Phylogenetic Cause of the Transposition of the . Testes in Mammalia : with Remarks on the Evolution of the Diaphragm and the Metanephric Kidney. By W . W o o d l a n d , University College, London.1 [Received March 3, 1903.] (Text-figure 57.) The subject of the descent of the testes in Mammalia, notwithstanding its bionomic interest, lias hitherto received but little attention from the phylogenetic standpoint; indeed, so far as I know, its interpretation has not hitherto been attempted. To exhibit the general nature of the problem and the apparent difficulty attending its solution, the following paragraph from Mr. Spencer's ‘ Principles of Biology' (vol. i. p. 573) may be quoted:-" But now let it be confessed that though all phenomena of organic evolution must fall within the lines above indicated, there remain many unsolved problems. Take as an instance the descent of the testes in the Mammalia. Neither direct nor indirect equilibration accounts for this. We cannot consider it an adaptive change, since there seems no way in which the production of sperm-cells, internally carried on in a bird, is made external by adjustment to the changed requirements of mammalian life. Nor can we ascribe it to survival of the fittest; for it is incredible that any mammal was ever advantaged in the struggle for life by this changed position of these organs. Contrariwise, the removal of them from a place of safety to a place of danger would seem to be negatived by natural selection. Nor can we regard the transposition as a concomitant of re-equilibration; since it can hardly be due to some change in the general physiological balance." I agree with Mr. Spencer that the descent of the testes can neither have been a change adaptive in nature, nor a result of the operation of natural selection; on the other hand, I must disagree with the statement that the phenomenon cannot be regarded " as a concomitant of re-equilibration," since, as will be shown, I attribute it to the direct action of the conditions of life found in the Mammalia. To ensure due appreciation of the significance of the following statements, it will be as well to here indicate the nature of the theory about to be advanced. Of possible causes effecting the transposition of the testes, there are two categories : (a) causes which as regards their effects on the organism mediately or immediately impart advantage, so supplying the requisite material for natural selection ; and (b) causes which in their effects on organisation bear no appreciable relation to either advantage or disadvantage. Since those of the former category are inconceivable in this connection, we are led to conclude 1 Communicated by Prof. E. A. M i n c h in , F.Z.S. P roc. Z ool. Soc.- 1903, V o l . I. No. X X I . 21 |