72 REPRODUCTION AND LIFE HISTORY. 



biogenetic law " reads : " Ontogeny, or the development 

 of the individual, is a shortened recapitulation of phylogeny, 

 or the evolution of the race." 



It is hardly necessary to say that the young mammal is 

 never like a worm, or a fish, or a reptile. It is at most 

 like the embryonic stages of these, and it may also be 

 noticed that, as our knowledge is becoming more intimate, 

 the individual peculiarities of different embryos are be- 

 coming more evident. " But this need not lead us to deny 

 the general resemblance. 



Moreover, the individual life history is much shortened 

 compared with that of the race. Not merely does the one 

 take place in days, while the other has progressed through 

 ages, but stages are often skipped, and short cuts are dis- 

 covered. And again, many young animals, especially those 

 " larvae " which are very unlike their parents, often exhibit 

 characters which are secondary adaptations to modes of 

 life of which their ancestors had probably no experience. 

 In short, the individual's recapitulation of racial history is 

 general, but not precise. It is seen rather in the stages 

 in the development of organs (organogenesis) than in the 

 development of the organism as a whole. 



(4) Organic continuity between generations. Heredity. 

 Everyone knows that like tends to beget like, that offspring 

 resemble their parents and their ancestors. Not only 

 are the general characteristics reproduced, but minute 

 features, idiosyncrasies, and pathological conditions, inborn 

 in the parents, may recur in the offspring. 



At an early stage in the development of the embryo the 

 future reproductive cells of the organism are often dis- 

 tinguishable from those which are forming the body. 

 These, the somatic cells, develop in manifold variety, and, 

 as division of labour is established, they lose their likeness 

 to the fertilised ovum of which they are the descendants. 

 The future reproductive cells, on the other hand, are not 

 implicated in the formation of the " body," but, remaining 

 virtually unchanged, continue the protoplasmic tradition 

 unaltered, and are thus able to start an offspring which 

 will resemble the parent, because it is made of the same 

 protoplasmic material, and develops under similar con- 

 ditions, 



