146 Dynamic Theory. 



the stimulus causing a peculiarity in the reaction of the muscle, tends 

 obviously to the setting apart or differentiation of certain muscle fibres 

 to certain forms of movement, and finally to new anatomical structure, 

 and hence limbs or locomotive organs on each side. The development 

 of the organs of the vertebrates being thus traceable to the bilateral ac- 

 tion of the original stimulus to motion and sensation, it is not strange 

 that the organs exist in pairs to so large an extent, especially those of 

 locomotion and sense. The two limbs on each side, the eyes and ears 

 in pairs, the double hemispheres of the brain, these are to be expected. 

 But the mouth and intestine is single from the highest to the lowest, 

 and that any of the products of either should be double is remarkable, 

 but may be explainable as the effects of later reaction of the outside of 

 the body upon the inside. The double parts, of internal origin, are the 

 kidneys, the ovaries and testes, and, later, the liver, lungs, double- 

 uterus, double male organ, and double nostril. The history of the de- 

 velopment of bilateralism is exceedingly curious, and its results to our 

 race are extremely important, as will appear. 



Some organs, after occurring as double in the lower animals, become 

 consolidated, or tend that way, in the higher. On the other hand, 

 some, which in some of the inferior forms are single, finally become 

 double in the superior. The cerebrum, which in. man is the principal 



FIG. 79. FIG. 80. 

 FIG. 79. Female generative organs of Ornithorhynchus. 



OV. Ovary. Kl. Cloaca. 



X. Non-functional right ovary. D. Intestine running down behind S and 



T. Oviduct. opening into the cloaca. 



U. Uterus double. Ur. Ureter. 



M. Mouth of uterus. F. Opening of ureter. 



S.Urogenital sinus or receptacle. H.Urinarv bladder. (Owen.) 



FIG. 80. Female organs of a Kangaroo lialmaturus. 



Ov. Ovary. T. Oviduct. 



U U. Uterus. o o. Mouth of utrnis. 



B. Coecum of vagina a pouch closed H. Urinary bladder opening into S. 



below. S. Urogenital sinus. ( (Jegenbauer.) 



