326 THE EVOLUTION OF ANIMAL BEHAVIOUR 
descendant of other reproductive cells ; and that, throughout 
a long ancestry, stretching back into the far past, there never 
occurs in the direct line of genealogical sequence, any highly 
differentiated cell, such as a gland-cell, muscle-cell, nerve-cell ; 
never, with certain reservations into which we need not enter, 
is found the representative of any tissue save that to which the 
reproductive function is restricted. In technical phraseology, 
the continuity of organic evolution is due to the continuity of 
the germinal substance. 
During embryological development the fertilized ovam— 
consisting of two fused fragments of this germinal substance— 
gives rise to a host of ordered and marshalled cells, which are 
divisible into two groups: the one forms the body with its 
muscles, bones, glands, digestive system, skin, sense-organs, 
nerve-centres, and so forth ; the other forms a reserve store of 
germinal substance, from which are derived the ova and sperms. 
The former take no direct share in reproduction ; they are off 
the line of continuous descent ; they die without issue. But 
they protect and minister to the reproductive function of the 
second group—the potential ancestors of the races to follow. 
But all instinctive and intelligent behaviour is the outcome of 
the orderly working of the nervous system, is initiated through 
sensory stimulation, and is executed by the motor organs ; and 
all the structural parts, through which such behaviour is pos- 
sible, belong to the body—that which dies without issue. 
How, then, can instinct and intelligence be inherited? In a 
sense they are not inherited. The nervous system which is 
their organic basis begets no heirs. But it is begotten of 
germinal substance, which not only produced the body of 
which the nervous system is a part, but also handed on, with 
that body, samples of the same germinal substance capable of 
reproducing a similar body and a like nervous system. Herein 
lies the basis of heredity. 
The stress of the struggle for existence falls upon the 
body ; and instinctive or intelligent behaviour is a means to 
its preservation in the struggle for existence. According as 
it survives or not, will the samples of germinal substance it 
