124 Dynamic Theory. 



respect to time. The testes of birds tire situated internally above the 

 kidneys near the lungs. During the mating season they attain an ex- 

 traordinary and enormous development. 



* FIG. 75. Testes of House Sparroiv at different periods. 



e ^ I. January. 



II. Middle of February. 

 g* III. Beginning of March. 



IV. Latter end of March. 

 O V.-Middle of April. 



(After J. F. Brandt.) 



The testes of the house sparrow which are one-twelfth 

 _.. of an inch in diameter in January become one-half an 

 inch in April, or 216 times as large. 



During the mating season the testes of Rodents come 

 down into the groin. The Shrews ( Tnsectivora ) have a 

 peculiar gland on each side just under the skin which at 

 the same season has an activity peculiar to itself in ex- 

 uding an odorous fluid. The hand of the frog has four 

 fingers and one of these it uses as a thumb. This thumb 

 in the male is furnished with a spongy swelling which en- 

 larges during, the breeding season to aid in grasping the 

 female. The down on the breasts of female birds disap- 

 pears during the season of incubation is not plucked out 

 but drops automatically to allow the bare warm skin of the mother to 

 come in contact with the eggs. Ordinarily ducks arid drakes differ 

 from each other in color, but immediately after the breeding season the 

 coloring of the drake changes to resemble that of the duck. This 

 change is not accompanied with the loss of the feathers which occurs at 

 another time the moulting season. 



The feathers of birds are in general renewed in autumn only, but in 

 some cases in the spring also. About the commencement of the breed- 

 ing season some birds undergo a change of color without moulting. 

 When this takes place as in certain G-ambets, an arctic bird (Totanus), 

 the coloring matter is often entirely absorbed previously to the autumnal 

 change of feather, and in some double moulting species, as the Golden 

 Plover, it commonly happens in spring that the coloring secretion tinges 

 the old feathers that are loose and ready to drop off, thus proving that 

 a circulation obtains in the pores of the feathers even up to the period 

 of their being naturally cast. ( Cuvier, P. 146.) There are many 

 other cases of special phenomena attending animal reproduction, the 

 periodical recurrence of which had its origin in the periodical recurrence 

 of the seasons and the movements of the planetary bodies. For the 

 reason that the seasons are different from ours on all the other planets it 

 is not possible that animal life on them should be like what it is here; 

 and as our own climates change, life will continue to be changed in the 

 future as it has been in the past. 



