456 THE EVOLUTION OF LIFE. 



remembering that these intrusions have been going on from 

 the beginning; we shall be prepared to find that the general 

 law of embryonic parallelism is qualified by irregularities 

 which are mostly small, in many cases considerable, and occa- 

 sionally great. The hypothesis of evolution accounts for 

 these : it does more — ^it implies the necessity of them. 



§ 130. The substitutions of organs and the suppressions of 

 organs, are among those secondary embryological phenomena 

 which harmonize with the belief in evolution but cannot be 

 reconciled with any other belief. Some embryos, during 

 early stages of development, possess organs that afterwards 

 dwindle away, as there arise other organs to discharge the 

 same functions. And in other embryos organs make their 

 appearance, grow to certain points, have no functions to dis- 

 charge, and disappear by absorption. 



We have a remarkable instance of substitution in the 

 temporary appliances for respiration, which some embryos 

 exhibit. During the first phase of its development, the* mam- 

 malian embryo possesses a system of blood-vessels distributed 

 over what is called the area vasculosa — a system of vessels 

 homologous with one which, among fishes, serves for aerating 

 the blood until the permanent respira-tory organs come into 

 play. Now since this system of blood-vessels, not being in 

 proximity to an oxygenated medium, cannot be serviceable 

 to the mammalian embryo during development of the lungs, 

 as it is serviceable in the embryo-fish during development of 

 the gills, this needless formation of it is unaccountable as 

 a result of design. But it is quite congruous with the sup- 

 position that the mammalian type arose out of lower verte- 

 brate types. For in such case the mammalian embryo, pass- 

 ing through states representing in a general way those which 

 its remote ancestors had in common with the lower Verte- 

 hrata, develops this system of vessels in like manner with 

 them. An instance more significant still is furnished 



by certain Amphibia. One of the facts early made familiar 



