48 ORGANIC EVOLUTION — PHYSICAL 



retrogress unless tliat tendency is checked by selection ; 

 for, as regards any trait, an individual may vary i'rom 

 liis parent primarily in two ways. He may vary from 

 him either towards the ancestry or away from it, i. c. lie 

 may either undergo retrogression or evolution; and, so 

 i'ar as we know, the chances are equal of his doing 

 either the one or the other. But if he vary away from 

 the ancestral ty)>e, it does not necessarily follow that 

 the variation will constitute an extension of the pre- 

 vious evolution. It may constitute a reversal of it, or 

 a divergence in an altogether new direction, and, there- 

 fore, in the absence of selection, the variation of the 

 offspring from the parent must tend on the whole to 

 bring about retrogression — a tendency which is checked 

 and reversed in an evolving sj)ecies only by a sufficiently 

 severe process of selection. 



Though each multicellular organism has its starting- 

 point in a unicellular organism (the germ cell), yet 

 nevertheless each "orm cell, countinir from the time of 

 the first evolution of the multicellular organism, when 

 all the cells were more or less germ cells, through suc- 

 ceeding generations, during which ever-increasing differ- 

 entiation in structure and sjDecialization in function took 

 place, must have become in one sense a more and more 

 complex entity, differing more and more from its ancestor, 

 the unicellular organism, in that it was the starting-point 

 of a more and more complex and heterogeneous multi- 

 cellular organism. In other Avords, each successive con- 

 jugating germ cell differed more and more from its 

 ancestor, the conjugating unicellular organism, not only 

 in that its non-conjugating descendants, however remote, 

 remained adherent in one mass, but also in two other 

 particulars. 



First, in that definite lines of its cell-descendants 

 multiplied at increasingly different rates, whereby were 

 produced differentiations in the shape of the whole 



