250 NATURAL SCIENCE. Oct., 1895. 



We know as a fact, that if a consumptive marries with one of 

 another family without that tendency, the chances are far better 

 for the children than if he married, say, a first cousin. The same 

 applies to insanity, and, as far as we know, to every constitutional 

 weakness. It follows from this that what we may term pathological 

 variations will tend to perpetuate themselves either by asexual repro- 

 duction, or, what comes to the same thing, by inbreeding. Crossing 

 preserves the progeny, but does so by approximating the progeny to 

 the mid-species. Whereas a scrofulous or a neurotic family inbred 

 for several generations would suffer absolute extinction, even two 

 such unfavourable types might manage to survive by crossing, for 

 some of the progeny would probably so far approximate the mid- 

 species as to be neither insane nor scrofulous. It is reasonable to 

 suppose that when we know more of pathology, we may have some- 

 thing definite to say about the weakness which causes the potato to 

 fall a prey to disease, and the infusoria to perish, unless crossed with 

 other races. In the meanwhile, we recognise that there is a weak- 

 ness, and we know of its existence in higher types, where we have 

 studied it more fully. 



May we not sum up the argument in a few words as follows ? 

 Variation is a function of all methods of reproduction, and the most 

 marked cases of variation are seen in the ontogeny of a single indi- 

 vidual, where the diverse tissues all arise by asexual divisions of a 

 single cell. Conjugation, or sexual reproduction, brings the progeny 

 to the mid-type, so that a thousand pugs, cats, mice, or men all more 

 or less resemble each other, and their qualities may be plotted out in 

 simple curves. This is not the case with asexual division, for here 

 each product parts company with and is uninfluenced by the rest of its 

 relations. This conservation of the type brought about by sexual 

 conjugation is an outstanding character of undeniable importance, 

 and preserves a species where the environment is uniform. 



J. B. Haycraft. 

 University College, Cardift". 



