,3,,. BIOLOGICAL THEORIES. 507 



Semper's experiments upon Limnea, though sinriilar, are less striking 

 examples of the direct effect of external conditions upon the develop- 

 ment of each individual. They show the size of a Limnea to be 

 as much dependent upon external conditions as was the taillessness 

 of Weismann's mice. 



The life-history of Phylloxera or of Aphis is an excellent example 

 of the direct effect of the environment, including food-supply and 

 temperature, upon the form of the offspring. That is, upon the form 

 and structure of the generation as yet unborn, through its effect upon 

 the internal influences determining the structure of the germs in 

 process of formation. In Phylloxera, four successive generations, all 

 different from one another, intervene between each two generations, 

 capable of amphigonous reproduction. More generations may 

 intervene — that depends upon external conditions — but these four, 

 all different from one another, are tolerably constant, and the 

 individuals of generation A in one year are like the individuals of 

 generation A in any other year, though unlike the ancestors of any of 

 the four next preceding generations. To the question " Why ? " my 

 answer is the same as in the case of the Rhabdoneniata. 



One more example will suffice. John Jones resembles his father 

 so closely that it is impossible to say which of two photographs 

 taken at similar ages is the portrait of John and which is that of his 

 father. This I hold to be due, not to the continuity of the spermatic 

 portion of the germ-plasm of two successive generations, but to the 

 reproduction, in two generations, of a complex set of influences which, 

 by their interaction and co-operation, have determined the form of 

 the father in one generation and of the son in the next, and these 

 influences I take to be both paternal and maternal, both internal 

 and, to a very slight degree, external also. 



All the cases which I have given as illustrations have one thing 

 in common. In every case I have referred to the occurrence of some 

 individuals like some other individuals belonging to an antecedent 

 generation, though the individuals resembled are in some cases 

 parents, in others grandparents or ancestors more remote, and in the 

 case of the termite soldier the individuals resembled are not ancestors 

 at all. In every case I hold the likeness between the two individuals 

 to be due to the likeness of the causes which have determined the 

 formation of those individuals. B is like A because its form and 

 structure have been determined by influences like those which have 

 determined the form and structure of A. When B is the direct 

 descendant of A, it is customary to call the phenomenon " heredity." 

 Rightly or wrongly, I hold the phenomenon to be of the same nature 

 in all cases, and to be simply an illustration of the general law : " Like 

 causes produce like effects." 



How the like causes. are produced, I shall endeavour to explain 

 in a future paper, as also the phenomena of atavism and variation. 



C. Herbert Hurst. 



