Recapitulation and Conclusion. 313 



it may be brought about not only by the various forms of 

 asexual reproduction, but also by the sexual union of 

 two reproductive elements, each of which is homologous 

 with the other cells of the body. 



In the lower animals and plants the cells which thus 

 unite with each other, or conjugate, are similar in form, 

 and probably in function also; but in all the higher 

 organisms the male cell is very different from the ovum 

 in form, size, and structure, as well as its mode of 

 origin. 



The present structure of each organism is the resultant 

 of two factors, which we may call adherence to type and 

 adaptation to new conditions, or if the use of terms with- 

 out teleo logical implications is desired, we may speak of 

 them as heredity and variation, or we may follow Haeck- 

 el and call them memory of past experiences, and percep- 

 tion of new relations. The precise terms to be used is a 

 matter of little consequence. The essential thing is the 

 recognition of the fact that each organism is the resultant 

 of two factors, and that evolution is two-sided. An 

 animal is what it is because it has the power to hold on to 

 the experiences and adaptations which fitted its parents/^ 

 for their place in nature, and the parents acquired those 

 peculiarities in virtue of their powers to gradually adjust 

 their structure and habits to their environment. 



This is the piorphological side of evolution. < Looking 

 at it from its dynamical or functional side, we notice 

 that each step in the process of advancement has been 

 reached by divergent specialization and by physiological 

 division of labor. Animals diverge from each other by 

 acquiring the power to occupy different fields, to procure 

 and use different kinds of food, to exist in different 

 media, etc., and the organs and tissues and cells of a 

 highly specialized animal or plant are adapted to perform 



