352 HISTORY OF THE HUMAN BODY 



atrophied and the new connections which have been added. 

 In the background may be seen the primitive cardinal 

 system of fishes, and perhaps in the minute caudal vein, even 

 a trace of the still earlier sub-intestinal system. Here, as 

 elsewhere, we receive the distinct impression of the constant 

 modification of old relations to fit new conditions, and we 

 see the numerous mechanical difficulties which are the in- 

 evitable result of such a process. Here and there, where 

 a difficulty is sufficiently great to interfere seriously with the 

 preservation of the race, it is overcome, if possible; if not, 

 the race dies out ; but generally the adaptation is fairly com- 

 plete, and, while we may never know of the countless forms 

 which were lost in the sifting process, those that survive 

 are not seriously handicapped by the circuitous paths through 

 which their organs have arrived at their final condition, and 

 the atrophied rudiments form no serious disadvantage to 

 the organism. 



As one example of the slowness of the adaptation where 

 the disadvantage is inconsiderable, we have the case of the 

 vena anonyma of the left side. Although the plan by which 

 all the venous blood is received upon the right side of the 

 heart is inaugurated by the amphibians, there is, in the an- 

 terior cardinal system, no anatomical recognition of this 

 throughout amphibians, reptiles, and birds, all of which still 

 possess symmetrical vena anonyma, and the left one is forced 

 to bring its blood over to the right side. First among the 

 mammals comes the formation of an obliquely placed trans- 

 verse connective, which allows the establishment of a true vena 

 cava anterior, and rectifies the slight mechanical disadvantage. 

 The mere fact that this condition continues so long without 

 readjustment suggests that the disadvantage must be ex- 

 ceedingly slight, far too inconsequent to come under the direct 

 control of Natural Selection; and the bettered condition in 

 these mammals cannot fail to suggest the result of mechanical 

 causes, operating continually for a long time, and always in 

 the same direction. 



The heart is in origin nothing but a localized portion of 

 a large blood-vessel, the walls of which develop a thickened 



