VII.] TRANSMISSION OF ACQUIRED CHARACTERS. 405 



the cause of its development into a bird, and we may similarly 

 maintain that the physical nature of a Thuja-shoot, and not the 

 influence of light, is the cause of the development of tissues 

 w^hich are characteristic of the species. In the development 

 of such a shoot the light plays precisely the same part which is 

 played by temperature in the development of a bird : it is one 

 of the conditions of development. 



There is nevertheless a difference between these two cases 

 in that the Thitja-shoot possesses the possibility of development 

 in two different ways instead of only one. The upper side of 

 the shoot can assume the structure of the under side and vice 

 versa, and this structural reversal depends upon the way in 

 which the light is thrown upon the shoot. But even if the 

 light causes the structural reversal, does this justify us in 

 assuming that the structure itself is also the direct consequence 

 of the influence of light ? I see no reason for rejecting the 

 supposition that the physical nature of part of a plant may be 

 of such a kind that this or that structure may be produced 

 according as this or that condition of development prevails. 

 Thus with stronger light the structure of the upper side of the 

 shoot developes ; with weaker light, the structure of the under 

 side. But this physical nature of the T/iuja-hud depends, like 

 that of a bird's egg, upon its phjdetic history, as we must 

 assume to be the case with the germs producing all individual 

 developments. It is therefore quite impossible to interpret the 

 reversal of the structure in the Thuja-shoot as the result of 

 modification produced by the direct influence of external con- 

 ditions. It is an instance of double adaptation — one of those 

 cases in which the specific nature of a germ, an organism, 

 or a part of an organism, possesses such a constitution that it 

 reacts differently under the incidence of different stimuli. 



An entirely analogous example of reversal occurs in the 

 climbing shoots of the Ivy, and is described in Sachs' lectures 

 on the physiolog3^ of plants. Such shoots produce leaves only 

 on the side directed towards the light, and roots (which are 

 made use of in climbing) only upon the opposite side. If how- 

 ever the position of the plant be altered so that the root-bearing 

 side is turned towards the light, while the leafy side is shaded, 

 a reversal occurs, so that from that time the former only 

 produces leaves, and the latter nothing but roots. In other 



