218 Dr. J. D. Macdonald on the Distribution of [Mar. 20, 



March 20, 1873. 

 Mr. GEORGE BUSK, Vice-President, in the Chair. 



The following communications were read : — 



I. cc On the Distribution of the Invertebrata in relation to the 

 Theory of Evolution/' By John D. Macdonald, M.D., 

 E.B.S., Staff Surgeon B.N., Assistant Professor of Naval 

 Hygiene, Netley Medical School. Received February 26, 

 1873. 



All organized beings exhibit both structural and functional conditions, 

 forming the grounds of comparison by which natural affinities, in smaller 

 groups, and points of difference in larger ones, are detected and esta- 

 blished hi systematic classification. 



General anatomical or physiological considerations in agreement are 

 usually of more importance than the harmony of siugle or special condi- 

 tions of either description ; and though structural characters, as a rule, 

 are superior to those of a functional nature, much may be learnt from an 

 arrangement founded on physiological principles alone. I have elsewhere 

 pointed out the deceptiveness of taking the habit of life as a guide hi 

 classification, though this is adopted by many zoologists ; for essentially 

 different types may live under precisely similar circumstances, or the 

 habit of life may be very different in the members of the same type. 

 Thus, if we look upon a pectinate gill for aquatic respiration, fluviatile or 

 marine, and the amphibious coincidence of this with a pulmonary 

 chamber, or the presence of the latter cavity alone in purely terrestrial 

 Gasteropods, as grouping characters, nothing can be more erroneous ; 

 for all these conditions of the respiratory system are to be met with in 

 unequivocal examples of the same group, anatomically defined, as in the 

 Nerite alliance, or that of Missoa for example. "Nevertheless animals- so 

 simple in their nature as the Protozoa may be distributed physiologically, 

 with some show of truthfulness, in the resulting scheme. 



Passing the leading types of the Protozoa in review, we notice that 

 the Gregarinidae alone are essentially parasitic in their habit of life, 

 obtaining nutriment from materials elaborated by other animals. All 

 the rest are therefore non-parasitic, deriving their sustenance from the 

 outer world. If we now consider the manner in which nutritious 

 matters are taken up and assimilated by these animals, we find that 

 some of them must subsist on organic substances in solution, which are 

 absorbed by the general surface of the body. Moreover we observe 

 that this takes place either indirectly through a more or less consistent 



