life: its nature, origin, and maintenance SCHAFEE. 509 



ESTABLISHMENT OF SEXUAL DIFFERENCES. 



After the appearance of a nucleus — but how long after it is im- 

 possible to conjecture — another phenomenon aj^peared upon the 

 scene in the occasional excliange of nuclear substance between cells. 

 In this manner became established the process of sexual reproduction. 

 Such exchange in the unicellular organism might and may occur be- 

 tween any two cells forming the species, but in the multicellular 

 organism it became — like other functions — specialized in particular 

 cells. The result of the exchange is rejuvenescence; associated with 

 an increased tendency to subdi^dde and to produce new individuals. 

 This is due to the introduction of a stimulating or catalytic chemical 

 agent into the cell which is to be rejuvenated, as is proved by the ex- 

 ])eriments of Loeb already alluded to. It is true that the chemical 

 material introduced into the germ cell in the ordinary j^rocess of its 

 fertilization by the sperm cell is usually accompanied by the intro- 

 duction of definite morphological elements which blend with others 

 already contained within the germ cell, and it is believed that the 

 transmission of such morphological elements of the parental nuclei is 

 related to the transmission of parental qualities. But we must not be 

 Ijlind to the possibility that those transmitted qualities may be con- 

 nected with specific chemical characters of the transmitted elements ; 

 in other words, that heredity also is one of the questions the eventual 

 solution of which we must look to the chemist to provide. 



AGGREGATE LIFE. 



So far we have been chiefly considering life as it is found in the 

 simplest forms of living substance, organisms for the most i)art entirely 

 microscopic and neither distinctively animal nor vegetable, which 

 have sometimes been grouped together as a separate kingdom of 

 animated nature — ^that of Protista. But persons unfamiliar with the 

 microscope are not in the habit of associating the term "life" with 

 microsco|)ic organisms, whether these take the form of ceQs or of 

 minute portions of lixdng substance which liave not yet attained to 

 that dignity. We most of us speak and think of life as it occurs in 

 ourselves and other animals with which wo are familiar; and as we 

 find it in the i)lants around us. We recognize it in these by the 

 possession of certain properties — movement, nutrition, groAvth, and 

 reproduction. We are not aware by intuition, nor can we ascertain 

 without the cmi)loyment of the microscope, that we and all tlie 

 higher living beings, whether animal or vegetable, are entirely formed 

 i)f aggi'cgates of nucleated cells, each microscopic and eacli possessing 

 its own life. Nor could we susj)ect by intuition tluit wliat we term 

 our life is not a single indivisible property, capable of being blown 

 out with a puff like the flame of a candle; but is the aggregate of the 



