392 ON THE SUPPOSED BOTANICAL PROOFS OF TilK 



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 which was intended to become the upper side 

 developes the spongy parenchyma which is characteristic of the 

 lander 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 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 which are character- 

 istic 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. 



