575.1 233 



The Fundamental Principles of Heredity 



IN the recent elaboration of the Theory of Descent, as first fully 

 published by Charles Darwin, two schools of thought have 

 arisen. The one, though professing discipleship pure and simple, 

 has laid extreme stress on the principle of Natural Selection, which 

 owes so much to Darwin, but has rejected his belief in the 

 internal tendencies of races to vary in adaptation to changed 

 surroundings ; while the other has attributed the greater share in 

 the transformation of species to the latter factor, and sent Natural 

 Selection in the background. The two most illustrious leaders 

 of scientific thought have been August Weismann on the one side 

 and Herbert Spencer on the other. Their debates have long since 

 obtained an audience among the cultured laity ; but while the 

 arguments are well known, some of the most important facts have 

 been rather taken for granted than fully stated and clearly co-or- 

 dinated even in the scientific press. I allude especially to the 

 coarser relations of the actual mechanism of reproduction and of 

 the act of transmission from one generation to the next of the 

 form which clothes on or assumes the parental characters. Such 

 an exposition as we have to make cannot be limited to the higher 

 organisms which are familiar to us in our daily life, for these are 

 complex elaborations ; while the primitive types, though still existing 

 abundantly, are only to be studied with the microscope. It is in 

 this field, hidden if not buried, that we must first labour, if we wish 

 to rightly understand the foundations of the wonderful superstructure 

 of the higher Organic Kingdoms. We shall endeavour to use as 

 few unfamiliar terms as possible, bearing in mind that the reader 

 has no Handy Atlas to help him in following the exploration of this 

 foreign country, with its outlandish names. 



Only two centuries ago the microscope revealed to mankind an 

 immense world of minute living creatures as well as the details of the 

 structure of the familiar Animal and Plants. Naturally enough 

 the early observers, or ' philosophers,' as they were then called, 

 inferred that these strange small creatures must have as complex 

 a structure as our own. They proceeded zealously to search 

 for, and sometimes to proclaim, the existence therein of brain, 

 heart, blood-vessels, etc., just like those of ordinary bird, beast, or 



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