RETROGRADE FORMS. 1 1 7 



Sufficient allowance is not made by many thinkers 

 for the infinity of difference even in structure, and 

 variety of change, possible in living forms, without the 

 production of two things alike, or any indication of 

 the merging of one set of forms into another. It 

 must not be forgotten for an instant that from such a 

 marvellous storehouse of facts as is placed at our 

 disposal in nature, we may with very little ingenuity 

 select certain series of facts in favour of several very 

 different general hypotheses ; and however conflicting 

 these may be with one another, it may not be possible 

 to disprove any one of them in the present state of 

 knowledge. The fact that masses of germinal matter, 

 derived by direct descent from cells of one of the 

 lower animals, may grow and multiply in man's 

 organism, and vice versa, might be adduced as an 

 argument in favour of the original common parentage, 

 countless ages back, of the predecessors of both ; but 

 there are, it need scarcely be said, facts and arguments 

 tending to a different conclusion, and these must not 

 be lost sight of in our attempts to ascertain the truth. 



It is not improbable that the germinal matter of 

 some of the lower, simpler plants and animals, when 

 exposed to altered conditions, may give rise to morbid 

 forms bearing a relation to their normal healthy 

 living germinal matter, similar to that which pus 

 bears to the germinal matter of healthy tissues, and 

 it is possible that in our observations upon the 

 lower forms of life we may be sometimes examining 



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