THE CAUSATION OF DISEASF. 87 



ancestry.* They are represented by the central darkly shaded 

 portion, d. The human embryo then takes on structural cha- 

 racters which were acquired during the amphibian stage. These 

 are represented by the shaded portion, c. It then assumes 

 characters acquired during the primitive mammalian stage, as 

 represented by b, and so on. The unshaded parts of the 

 diagram represent characters which belonged to the different 

 ancestors of man, but which no longer appear in the human 

 cycle, f 



It will be convenient, while treating of this subject of 

 structural stability or fixity, to consider, first, the tendency 

 of structural characters, actually possessed by the individual, 

 to disappear in the inverse order of their age — a, ancestral, 

 or &, individual ; and, secondly, the tendency of structural 

 characters, of recent ancestral date, and possessed by either or 

 both parents, not to appear in the offspring — this tendency 

 being in proportion to the ancestral youth of the structural 

 character. 



We may accordingly frame the following propositions : — 

 la. Characters which the individual possesses by virtue of 

 heredity, tend, under disturbing influences, to disappear in the 



* The line from X to A should be very much longer than here represented ; 

 for the distance in point of time between X and A is infinitely greater than 

 that between A and E. 



f It must be borne in mind that the various changes which take place in 

 development (ontogeny), are not an exact epitome of the several ancestral 

 forms of life, for not only have numberless ancestral characters been dropped 

 as indicated in the above diagram, but there is an actual vitiation in the pro- 

 cess of development, so that it no longer gives us a perfect ancestral history 

 of the organism. So far as the process of embryological development is an 

 exact reproduction of ancestral life, it is termed by Haeckel, Palingenesis, or 

 inherited history (7raXt^= reproduced). So far as it is not such an exact re- 

 production, he terms it Kenogenesis = vitiated history, (icevos = strange, 

 meaningless). This aberration of development, as we may in a manner term 

 it, has reference («) to the period at which the different organs appear, some 

 developing sooner, and others later, than in the ancestral forms of life ; thus, in 

 man there is an acceleration in the appearance of the gill openings, of the 

 brain and eyes ; and it has reference (b) to position — i.e. , an organ may develop 

 from a different tissue from that in which it originally developed. Thus, the 

 sexual organs — both male and female— in man originate from the mesoblast, 

 while in the lower animals the male sexual organs originate from the epiblast ; 

 the female from the hypoblast. It would appear that in such cases certain 

 embryo cells have migrated from one germ-layer to another. (See, on this sub- 

 ject, " The Evolution of Man," by Ernest Haeckel, vol. i. p. 10, London, 1879.) 



