4C4 O.V SUPPOSED BOTANICAL PROOFS OF [VII. 



experience which belong to botany and zoology. It will be useful 

 to orain a clear idea of the differences which are thus caused. 



Detmer first alludes to the dorso-ventral structure of the 

 shoots of Thuja occidentalis, chiefly shown in the fact that the 

 upper sides of these shoots contain the green palisade cells, 

 while the under sides which are turned away from the light 

 possess green spheroidal (isodiametric) cells. If the branches 

 of Thuja are turned upside down and fixed in this position 

 before the production of new shoots, it is found that the 

 anatomical structure of the latter, when developed, is reversed. 

 The side of the shoot which was destined to become the under 

 side, but which was artificially compelled to become the upper 

 side, assumes the structure of the upper side and developes 

 the characteristic palisade parenchyma ; and on the other 

 hand, the under side w^hich was intended to become the upper 

 side developes the spongy parenchyma which is characteristic 

 of the under side. From these facts Detmer concludes that 

 the dorso-ventral structure of the shoots of Thuja has resulted 

 from the continual operation of an external force, and that the 

 light must be considered as the cause of the structural change. 



But such a conclusion obviously depends upon a confusion 

 of ideas. No one will doubt that the light was the stimulus 

 which led to the reversal of the structures in the shoot, but this 

 is a very different thing from maintaining that it was the cause 

 which conferred upon the Thuja-shoot the power of producing 

 palisade and spongy parenchyma. When a phenomenon only 

 occurs under certain conditions, it does not follow that these 

 conditions are the cause of the phenomenon. A certain 

 temperature is necessary for the development of a bird in the 

 egg, but surely no one will maintain that the temperature is 

 the cause of the capacity for such development. It is obvious 

 that the egg has acquired the power of producing a bird chiefly 

 as the result of a long phyletic course of development which 

 has led to such a chemical and physical structure in the egg 

 and the fertilizing sperm-cell, that after their union and 

 development, a bird, and only a bird of a particular species, 

 must be produced. But of course certain conditions must be 

 fulfilled in order that such development may take place ; and a 

 definite temperature is one of these conditions of development. 

 Thus we may briefly say that the physical nature of the egg is 



