654 XrV. PYRROLE PIGMENTS IN EVOLUTION 



As Wiggles worth (3081) has pointed out, the hematin compounds 

 of hemichrome nature which are found in hemolymph and salivary 

 glands of certain blood-sucking insects are derived from the hemo- 

 globin of host and prey and are apparently metabolically functionless. 

 The fact, however, that they are passed on in the egg yolk indicates 

 that they are nevertheless of biological significance. Perhaps they 

 serve as reserve food for the construction of the hematin enzymes 

 before the nymph receives blood. If this is correct, these animal 

 species require hematin as a growth factor in the same way as the 

 flagellates or Hemophilus influenzae (cf. Chapter XIII, Section 3.2.2.). 

 Perhaps the urechrome found in the eggs of the pacific marine worm 

 Urechis caupo serves a similar purpose {134-8). 



Other hematin compounds may be unsatisfactory cul-de-sacs of 

 evolution. Helicorubih does not appear to possess any physiologic 

 function and may be considered an excretory product, particularly 

 in view of the fact that Helix does not require hematin for the forma- 

 tion of its oxygen carrier (hemocyanin). It should be noted, however, 

 that hemocyanin animals possess the cytochrome system and that 

 some of them have even myohemoglobin in their heart and adductor 

 muscles {125). 



The hemoglobin of root nodules poses an interesting evolutionary 

 problem, since neither the host plant nor the bacteria (Rhizobium) 

 alone are able to produce it. Its physiologic role is perhaps con- 

 nected with nitrogen assimilation {cf. Chapter IX, Section 5.2.).* 



In conclusion it will have been observed that the tentative hypoth- 

 eses put forward in the earlier sections of this chapter by no means 

 solve the problem of the origin and evolutionary role of the pyrrole 

 pigments. The present state of knowledge- is insufficient to make this 

 possible. It demonstrates, on the contrary, the difficulties which face 

 the worker in the field of chemical aspects of evolution, where the 

 guidance of the fossil record is denied him in the most crucial stages 

 of his search. It would seem that further progress will be slow, and 

 will derive its main impetus from further comparative studies of 

 primitive types of organisms, as representative, at least in approxi- 

 mation, of those which lived in the, even geologically, distant past. 



* The hemoglobin found occasionally in some species of Daphnia (Fox, 937aa) does 

 not appear to have an essential physiologic function, although, like the hematin 

 compounds mentioned above, it is passed on in parthenogenetic eggs. Even the 

 chlorocruorin or erythrocruorin of serpulimorphid worms does not appear to be 

 essential (Fox, 937 a). 



