284 HABIT AND INTELLIGENCE. [chap. 



of all vegetables. ISTow, there is no possibility of making 

 any confusion between an animal and a rose-busb, or an 

 apple-tree. Science is needed, not to inform us of the 

 difference, but to tell us of the fundamental resemblance 

 that all plants have to all animals. 



The same is true of the lowest of the unmistakeably 

 vegetable classes, Thallogens. This class consists of three 

 "Va^*^^^ orders — Algae, Lichens, and Fungi. These in their highest 

 Lichens' genera are distinct enough, but of some of their lower 

 and Fungi: genera, as Protococcus, it is nearly impossible to say of 

 which of the three orders they are members ; and, indeed, 

 on my view, they do not necessarily belong to one order 

 more than to another. They may belong to that simple 

 original type out of which Algae, Lichens, and Fungi have 

 all been developed by differentiation, 

 of fishes The next instance I shall mention is the connexion 



breathing ^^tween the fishes and the air-breathing Vertebrata. The 

 Veitebrata. air-breathing Vertebrata are on the whole much higher in 

 the scale of organization than the fishes ; but the Perenni- 

 branchiate Batrachians, which are the lowest of the air- 

 breathing Vertebrata, approximate, not to the highest, but 

 to the lowest of the fishes. 



There are, however, some exceptions to this law, which 



may probably be regarded as parallel cases to that of the 



metamorphosis of Cirrhipedes, in which, as we have seen, 



Eetrograde the change is retrograde — that is to say, from a higher to 



c ange. ^ lower form. One of the most remarkable instances 



Acari. of this kind that I know of is that of the Acari, or 



mites, which are the lowest members of a class which I 



believe to be descended from worms, and yet do not, 



at least normally,^ present the slightest approximation 



to the worms. This subject must be explained in some 



detail. 



^ I say "at least iwrmally," because there are instances of Acari pre- 

 senting a form which may be due to reversion to their ancestral worm- 

 character. " Mr. Charles Robertson, Demonstrator of Anatomy in the 

 University of Oxford, has lately described a form of Acarus found inside 

 pigeons, chiefly among the connective tissue of the skin, the large veins 

 near the heart, and on the surface of the pericardium. In some respects 



