266 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



sense of hearing or of smell, or were to suddenly acquire a colour 

 different from that which it now possesses, it would become in- 

 capable of existence as a species, and would soon die out. The same 

 result would probably occur if any of its internal organs, such as 

 the lungs or the liver, were suddenly modified. Perhaps single 

 individuals would still remain capable of existence under these cir- 

 cumstances, but the whole species would suffer a certain decline 

 from the maximum development of its powers of resistance, and 

 would thus become extinct. The sudden transformation of a species 

 appears to me to be inconceivable from a physiological point of 

 view, at any rate in animals. 



Hence the transformation of a species can only take place by the 

 smallest steps, and must depend upon the accumulation of those 

 differences which characterise individuals, or, as we call them, 

 ' individual differences.' There is no doubt that these differences 

 are always present, and thus, at first sight, it appears to be simply 

 a matter of course that they will afford the material by means of 

 which natural selection produces new forms of life. But the case is 

 not so simple as it appeared to be until recently ; that is if I am 

 right in believing that in all animals and plants which are repro- 

 duced by true germs, only those characters which were potentially 

 present in the germ of the parent can be transmitted to the 

 succeeding generation. 



I believe that heredity depends upon the fact that a small portion 

 f the effective substance of the germ, the germ-plasm, remains 

 unchanged during the development of the ovum into an organism, 

 and that this part of the germ-plasm serves as a foundation from 

 which the germ-cells of the new organism are produced * . There is 

 therefore continuity of the germ-plasm from one generation to 

 another. One might represent the germ-plasm by the metaphor of 

 a long creeping root-stock from which plants arise at intervals, 

 these latter representing the individuals of successive generations. 



Hence it follows that the transmission of acquired characters is an 

 impossibility, for if the germ-plasm is not formed anew in each 

 individual but is derived from that which preceded it, its structure, 

 and above all its molecular constitution, cannot depend upon the 

 individual in which it happens to occur, but such an individual 



1 Compare the second and fourth of the preceding Essays, ' On Heredity ' and ' The 

 Continuity of the Germ-plasm as the Foundation of a Theory of Heredity.' 



