60 THE PRINCIPLES OF HEREDITY 



reversion but not the regression. When the reversion affects 

 only part of a structure, and is disguised by progressive 

 changes in other parts, we recognize the regression, but not 

 the reversion. When, however, it appears in a simple form 

 as when the descendant of a line of individuals, each of whom 

 had six digits on one hand, reverts to a remoter ancestor who 

 had only five we see clearly that regression and reversion 

 are identical. 



99. Progressive variations, also, may occur at any stage of 

 the development. N., after recapitulating the whole of M/s 

 development of the structure, may add to it. His variation 

 may then be compared to an increase of growth at the 

 point of a twig. Or the variation may not appear at the 

 end of his development, but during the course of it like a 

 lateral bud. Thus it may appear at the point H. reached 

 at the end of his development, not at the point M. reached. 

 In other words it appears during embryonic life at that 

 stage which represents H. It is these variations which 

 occur during development that have rendered plants and 

 animals complex, and the study of heredity difficult and 

 obscure. In the illustration we considered just now we 

 studied a structure which we imagined as evolving in a series 

 of individuals, A. ... M. in one direction only as it were 

 in a line. But every structure has breadth and thickness as 

 well as length. During development a thousand, or a million, 

 variations may appear on the surface or in the substance of 

 the structure. Any one of these, if favoured by Natural 

 Selection, may grow to a size and importance far exceeding 

 that of the parent and structure just as a twig, if favoured 

 by sunshine, may eventually exceed the rest of the branch on 

 which it grows. Every variation, when once it has started, 

 may be looked upon as a structure capable of independent 

 variation in an almost infinite number of directions, regres- 

 sive and progressive. As a consequence every species of plant 

 and animal is in a condition of continual flux and change. 

 As a whole the likeness to the parent is well preserved. 

 But in minutiaB there is an immense amount of variation. 



100. If we think of a complex plant or animal or even of 

 a single structure as a whole we are unable, as a rule, to 

 reason clearly. We then see progressive and regressive 

 variations occurring in multitudes, and we become confused 

 by the immense mass of details. We may perhaps be able 

 to say that the organism, or organ, as compared to the parent, 

 has progressed or regressed in the sum of its characters ; but 

 we cannot distinguish the nature, the meaning of progression 



