12 HEREDITARY CHARACTERS 



can only produce and regulate the production of the cells 

 that go to build up its own particular kind of animal or 

 plant. Moreover, it can only produce the different kinds 

 of cells in a particular order. Such an assumption is in 

 exact accordance with such facts as are known to us. We 

 are so used to seeing like produce like in the case of animals 

 and plants, that we should be just as surprised if a cat 

 produced a puppy, as if the sun were to rise in the west ; 

 one event is indeed as likely to happen as the other. 



All these theories have been evolved with the object of 

 explaining how it is that the fertilised ovum of any particular 

 kind of animal or plant produces an organism similar to 

 those from which it was derived, no matter how complicated 

 that organism may be. As all multicellular organisms arise 

 from a single cell, and are produced by the division of this 

 single cell into two more, each of these two dividing again, 

 and so on until the organism is completed, it appears certain 

 that something must be present in the fertilised ovum which 

 is handed on from it throughout the succeeding generations 

 of cells, which regulates and limits the lines along which 

 development takes place. The questions remain as to 

 whether the ovum itself consists entirely of this substance 

 and of nothing else ; whether this substance is distributed 

 throughout the ovum, with the main part of the ovum as 

 a medium in which it lies ; or whether it is limited to some 

 particular part of the ovum, and of the cells subsequently 

 produced from the ovum. As in any case this substance 

 must be handed on from the fertilised ovum to the two cells 

 into which the ovum divides, and from them on to the 

 subsequent generations of cells produced from them, it seems 

 desirable to consider such facts as are known about the way 

 in which cells divide, in order to see whether in this process 

 there is any suggestion as to the existence and history of 

 such a substance. 



