38 



Now, it matters not the degree of importance that we give to Natural 

 Selection as a principle in organic evolution, it does not appear that we 

 can regard it as furnishing a final solution of the phenomena to be ex- 

 plained. This objection has been justly urged: Natural selection acts 

 only on characters which have been already produced and have become 

 either useful or hurtful. By what means have they been produced? 

 Before they can be selected they must exist; what principles or forces 

 gave them their existence? It has been urged that if there are intiuences 

 that can bring characters up to the stage where selection can begin to act 

 on them, the same intiuences might continue to perfect them. Darwin 

 saw the situation clearly. He says, in his " Descent of Man: " "With 

 respect to the causes of variability, we are in all cases very ignorant, but 

 we can see that in man, as in the lower animals, they stand in some 

 relation with the conditions to. which each species has been exposed dur- 

 ing several generations." He then mentions, as some of the probable 

 causes of change, the direct and definite action of changed conditions, the 

 efiects of increased use and disuse of parts, arrests of development, corre- 

 lated variations, &c. Under such circumstances it becomes a legitimate 

 subject of inquiry what those fox'ces and conditions are which have been 

 active in initiating changes in organisms, and what effect, if any, Natural 

 Selection has had in perpetuating and accumulating these new characters 

 and of repressing others. 



One of the most recent and most thoroughly elaborated attempts to ac- 

 count for the variations of organisms is that of Dr. Aug. Weismann. It 

 is presented in a series of lectures delivered between the years 1880 and 

 1890. The fundamental idea of his theory he has denominated "the con- 

 tinuity of the germ-plaf<vi." All except the lowest animals are produced 

 from eggs, which are essentially cells. When the egg is fertilized, it de- 

 velops into an embryo by a process of division which leads to the pro- 

 duction of an immense number of cells. These, becoming more and more 

 differentiated in definite ways, form the tissues and organs of the adult 

 being. Thus, from a simple egg there arises an animal which inherits 

 the general features of the parent and even many of its minor peculiari- 

 ties of form and habits. At some time during embryonic development 

 there are separated from the other cells of the organism certain cells 

 which in due season develop into eggs, as a provision for the continua- 

 tion of the species. It appears hitherto to have been assumed that the 

 materials of these eggs, or germ-cells, is derived by some process of trans- 



