58 J. B. CLELAND. 
first sight deemed but one, nearly always show not one slight 
difference in detail alone, but usually several, apparently 
unrelated. <A striking instance is seen in the ease of the 
two Bilharzia trematode worms affecting man in Egypt. 
For years controversy raged as to whether one species or 
two were responsible for the lesions found in man. This 
was due to the fact that, though the adults of each seemed 
the same, in some cases terminal-spined eggs were found 
alone, in others lateral-spined. We know now, through Lei- 
per’s researches, that there are two species, Bilharzia he- 
matobium and B. mansoni, that the difference in positions 
of the spines is due to the difference in species, that the 
intermediate molluscan hosts belong to different genera— 
Bullinus in the former, Planorbis in the latter, and that the 
cercarie differ in minor morphological details. It would. 
seem as if, in the differentiation of species, not one factor, 
but several, alter together. Though these factors may not. 
appear to be related to each other, it may be that they all 
have a common origin in a single mutation in the germ- 
plasm. In human beings we do not know why certain blue 
sclerotics of the eye should be associated with fragility of 
the bones. The two are so constantly found together that 
their origin from a common factor seems clear. For these 
reasons, in the case of the Shepherd’s Companion, I would 
scrutinise the bird closely to look for collateral evidences of 
change, were a race met with departing in mental charac- 
teristics from the type—and I am of opinion that I would 
not look in vain. 
We see, therefore, that besides morphological characters.. 
chemical ones and mental traits may all with advantage be 
considered in drawing up the specific description. Now 
just as important as these is the way the species, be it ani- 
mal or plant, reacts in disease. In Western Australia there 
are two species of Eucalyptus—H. margimata, the jarrah, 
