36 ANIMAL LIFE AND HUMAN PEOGRESS 



probably I sLould be repeating what is already well known 

 to most of you. 



I only propose to state as briefly as possible some of tbe 

 conclusions of biological science which seem to me so well 

 founded and so incontrovertible that they must be taken 

 into account in all our reasoning and in all our practice regard- 

 ing living things, particularly living animals, including man 

 himself. 



In doing this I may take as a text the quotation that I have 

 read from the concluding passages of the Origin of Species, 

 and need hardly go further afield. 



In the first sentence there is reference to the dependence 

 of animal (and vegetable) forms on one another. I may 

 amplify this by another quotation {Origin of Species, p. 60) : 

 " The structure of every organic being is related, in the most 

 essential yet often hidden manner, to that of all other organic 

 beings with which it comes into competition for food or 

 residence, or from which it has to escape or on which it preys." 



This is a statement of the universal phenomenon of adapta- 

 tion among living organisms. It is a phenomenon which 

 attracted the attention of the earhest naturalists. As the 

 quotations show, it attracted Darwin's special interest, and 

 it forces itself more and more on our attention as our know- 

 ledge of zoology increases. 



There is no more fascinating branch of zoology than the 

 study of adaptations ; their complexity and the fineness of 

 adjustment displayed by them are marvellous, and it is a point 

 well worthy of remark that dehcacy and complexity of adapta- 

 tion are no less strikingly exemplified in the simplest and most 

 lowly organised animals than in the highest and most complex. 

 An example, now well known, is furnished by the malarial 

 parasites. There are three kinds which produce malaria in 

 human subjects, and their life-history is divided into two 

 cycles : — an agamic cycle, in which the parasite multiplies 

 very rapidly in the human blood ; a gamic cycle, followed by 

 production of the so-called sporozoites, which takes place in a 

 mosquito. When an infected female mosquito sucks blood from 

 an uninfected human subject, it injects a number of sporozoites 



