Differentiation. 279 



organs, have been given in former chapters, and we shall meet with others. 

 Some of the other sort will now be given. All our rudiments are 

 survivals in man of organs or forms which were permanent and of use 

 in some of our animal ancestors, but useless to us. As seen in chapter 

 5, we carry a liberal stock of these useless and played-out appendages 

 about with us. It occasionally happens that the development is ar- 

 rested in regard to a particular part, but that such part, instead of re- 

 maining a useless rudiment, proceeds to perfect itself and to become 

 functional at a lower stage, as it is in some lower animal form. But 

 there are other classes of phenomena still, some of which fire denom- 

 inated anomalies, others monstrosities, which are to be explained on a 

 different principle. If a man should possess a certain organ not con- 

 structed after the human pattern, but like the corresponding organ in a 

 cow, and such organ were functional and -served the purpose, it would 

 be called anomalous, and yet it might not be strictly a case of arrested 

 development. In the course of embiyonic life we are constantly pass- 

 ing points at which the embryonic life of some other animal, which thus 

 far has been identical with ours, diverges from our own and takes a 

 different development. For example, a portion of the system of blood 

 vessels of the vertebrates is developed from the original five gill arches. 

 (See fig. 27.) But in none of the higher animals are all of these arches 

 utilized, the birds adhering to and developing one part of the original 

 form, and the mammals another, while the parts not utilized are sup- 

 pressed. (See figs. 28 and 29.) Different families of mammals differ- 

 entiate this development still further ; the normal form of these blood 

 vessels being different in man from that which obtains in cows or whales. 

 When in the course of human development the embryo has reached one 

 of these points of divergence which lead off into the developmental path 

 of a dog, for example, we may confidently assert that nothing but its 

 environment with its particular set of influences, makes its further prog- 

 ress certainly human, and insures it against switching off upon dog 

 development, as relates to that special part. But the fact is that the 

 environing influences of the human embryo are so nearly those of the 

 other mammal embryos that they do not always insure a rigidly human 

 development, the switching actually does, in a great number of cases, 

 take place, and the man becomes, so far as the involved parts are con- 

 cerned, a dog, a whale, or a bat, &c. 



All this is readily explainable on the theory that the mammal germ 

 is a crystalloid, and, when immersed in a proper solution, tends to de- 

 velop along a particular line of crystallization. But its complexity and 

 numerous subordinate polarities render subordinate modifications possi- 

 ble when the environing solution presents slightly modified conditions. 



An excellent article by Dr. Francis J, Shepherd, in Popular Science 



