ANIMAL EVOLUTION 17 



contain many different kinds of ids, representing the 

 contributions of many different ancestors. All this would 

 take me far from my main subject, and would require 

 many hours to set forth in sufficient detail. 



It is sufficient if I have said enough to show how 

 widely this theory differs from that of Herbert Spencer, 

 and what very different consequences must be deduced 

 from it. 



According to Spencer, the material of the germ^ out 

 of whi ch th e mSividual is develo ped, has indeed its o wn 

 properties which impose a limit upon and determine 

 "general direction ot the~developinent ; but within 



ieTfmtgri ahis - hi^hly p trP^i^, -^-^ri it o<;c^^m ~Pt^ 



its ultimate form and ^qu^litipg ir^ ]-f^c;pnnt;p iq Pvfprnal 



and i nternal forces ac ting_r>T]t if fhronghnnf thp wh''^^^ 

 co urse of ontogeny .^ The material, being of a certain 

 kind, can only react in a certain way to external and 

 internal forces, and so gives rise to a specific form ; but 

 as the forces must always be slightly different, the 

 reactions must always be slightly different, and thus 

 variations arise which are perpetuated in the germ-cells 

 separated off for the maintenance of the race and the 

 building up of the next generation. 



According to Weismann and his followers, the material 

 of the germ is already endowed with all the properties 

 of the adult ; the development of the individual is 

 pre-determined by the qualities of the germ ; the germ- 

 plasm is only slightly plastic, and such changes as are 

 impressed on the soma by the action of incident forces 

 perish with the soma, and are not incorporated into the 

 germ-plasm that is to give rise to future generations. 



In the words of Wilhelm Roux, the development of 

 the individual may be likened to a mosaic-work ; the 

 substances out of which the picture is to be formed are 

 there beforehand : the picture is formed by sorting out 

 these substances and combining them in a definite order. 



If now we inquire into the evidence in favour of one 



c 



