54 The Nature of Biological Diversity 



specialized substances produced by the tubercle organisms and the 

 blood group polysaccharides, have much in common. Cellulose oc- 

 curs widely in plants and occasionally in animals, especially in the 

 tunicates, but we know little about its fine structure. Chitin occurs 

 in many invertebrates, but even its structure is still not precisely 

 known. Galactogen, a major constituent of the eggs and albumin 

 glands of certain snails, is uncommon; indeed it is uncommon if only 

 in that about one in every seven of the constituent galactose units 

 consists of the L- instead of the usual D-isomeride (Bell and Baldwin, 

 1941). Glycogen, the commonest of animal polysaccharides, some- 

 what resembles the amylopectin component of the plant starches yet 

 differs somewhat according to its source, but the differences are not 

 chemically very remarkable. Glycogens from various sources give 

 different colors with iodine, ranging from zero to the well-known port- 

 wine coloration, probably reflecting differences in chain length and 

 degree of branching. However, it is difficult to know what chain length 

 means in molecules which are continuously changing as new units 

 are constantly added or removed. These are essentially dynamic, ever- 

 changing, and thoroughly recalcitrant molecules. 



So far we have mainly considered some variations on some one or 

 other of a few general themes. But it happens from time to time that 

 the comparative biochemist comes across substances that appear at 

 the time of their discovery to be uniquely confined to particular 

 groups or species. Galactogen, for example, is known to be present in 

 the eggs and albumin glands of Helix pomatia, H. aspersa, and 

 Limnaea sp. and might have been thought to be a characteristic 

 feature of gastropod mollusks, but it could not be detected in the eggs 

 of Aplysia punctata, a marine gastropod, nor in those of the cephalo- 

 pod, Sepia officinalis (Baldwin, unpublished). Maybe it will turn out 

 to be a feature of pulmonates as opposed to operculates, but too few 

 species of either have as yet been studied. 



The trouble here is, as has often been pointed out, that comparative 

 biochemistry has not in the past been comparative enough, and, until 

 it becomes so, it behooves us to be wary of associating this or that 

 substance too closely with some one or other particular group of 

 animals. A recent example of this is the case of homarine. First dis- 

 covered in lobster muscle by Hoppe-Seyler (1933), it has now been 

 found to be a very widely distributed constituent of invertebrate 

 materials and may, for all we know to the contrary, occur in verte- 

 brate tissues as well. It had indeed already been found in two other 

 phyla, viz., in Arbacia sp. by Holtz, Kutscher, and Thielmann (1924) 



