BASIC ASSUMPTIONS 9 



indicate any close phylogenetic relationship. Thus though many 

 Crustacea have haemocyanin no biochemist or physiologist would 

 suggest taking Daphnia out of the Crustacea because it possesses 

 haemoglobin. The role of blood pigments has been much studied 

 and in particular we accept the varied way in which they are dis- 

 tributed throughout the animal kingdom. Even here we do not 

 always know their function ; thus we find haemoglobin in the root 

 nodules of leguminous plants (Keilin and Wang 1945), where its 

 precise function is as yet not known. Plants, however, seem 

 capable of synthesising many substances that are often regarded 

 as " mammalian " compounds. Nettle stings contain acetyl 

 choline, 5-hydroxytryptamine and histamine, and it is probable 

 that these have been independently developed by the higher 

 plants. 



There are in the world but some ninety elements, and of these 

 only a few such as carbon, nitrogen, oxygen, hydrogen, phosphorus 

 and sulphur appear capable of forming natural monomers and 

 polymers. It is therefore not surprising that these elements are 

 united to form compounds such as citric acid or 5-hydroxy- 

 tryptamine in widely separated plants and animals. Such a 

 synthesis might have occurred independently on many occasions 

 by trial and error. It should be remembered that there is no 

 Patent Law in the natural world, and though one can simplify 

 the situation by use of William of Occam's razor, the careless 

 use of such a weapon can at times be suicidal. 



Our ignorance is even greater in other biochemical fields, yet it 

 is often stated that all protoplasm shows the same fundamental 

 biochemical systems. The most quoted example is the way in 

 which protoplasm oxidises carbohydrates to liberate energy. This 

 release of energy is obtained through two biochemical cycles, 

 the glycolysis cycle (Embden-Meyerhof) and the tricarboxylic 

 acid cycle (Krebs). Many of the chemicals present in these two 

 cycles have been found in bacteria, protozoa, plants, lower 

 metazoa, birds and mammals, and because some of the ingredients 

 are present it is assumed that the whole system is present. The 

 argument then runs that because the system is very complex, it 

 would be too much to expect that each group developed this 

 complex system independently and so protoplasm everywhere 

 must have had a common origin. 



