Il6 MARINE BIOLOGICAL LABORATORY. 



ovum to develop by introducing spermatozoa. A new 

 nucleus was then formed by these, and the ovum de- 

 veloped into a free-swimming larva. 



If before sexes were fully established, two germ-cells 

 of different origin were to unite, the embryo would have 

 a dual origin, and hence would differ from both parents. 

 If the variation thus introduced were favorable, sur- 

 vival of the fittest would preserve it. The germ-cells 

 of this form would, -of course, inherit the tendency to 

 unite with another germ-cell. In this way natural selec- 

 tion would establish sex, to ensure the combining of two 

 germ-plasms in each individual. Without such combina- 

 tion no variation in the Metazoa could occur. Hence 

 all sexual characters — even to the higher mental attri- 

 butes — are due in their origin to the necessity of 

 establishing variation. Variation, then, is a consequence 

 of the union of two sexual germ-plasms, and not some- 

 thing inherent in protoplasm. 



At first thought it seems strange that variations are 

 not more numerous and marked than they are ; that for 

 thousands of years the germ-plasm of every species of 

 animals has undergone so Httle change that each species 

 still retains all its specific characters. It would seem 

 that if animals are not suited to the environment, they 

 are not modified, but exterminated. Yet when advan- 

 tageous variation has occurred, and the race has been 

 preserved by natural selection as a different species, 

 this must have been brought about very slowly. It is 

 thus evident that the plasm is very conservative and 

 difficult to change ; that the conservative forces far out- 

 weigh all tendencies to vary. 



Now to return to the question whether death is 



