INTRODUCTORY 3 



mones". This analysis of self -maintaining activity has to be applied, 

 of course, to plants as well as to animals, hence the long discus- 

 sion of Plant Physiology. 



But the marvellous efficiency of the organism's self-maintaining 

 and self-repairing has its counterpart in its power of giving rise to 

 other organisms in its own likeness. This line of functioning stands 

 in some ways by itself, and, though its apartness has been over- 

 emphasised in the past, it is practically necessary to devote several 

 chapters of this book to the physiology of Reproduction and Sex. 

 For all this has to be traced upwards from the simple divisions and 

 unions of unicellular organisms, and thence onwards to the often 

 strange and intricate life-secrets of higher forms, and at length to 

 their uttermost veiling — yet now unveiling — ^within the apparently 

 simple flower, conspicuous though, as in the lily, its sexes long 

 seemed. On the animal ascent we come to the elaborate courtships 

 of many birds, the various conjugal ways of mammals, and the 

 parental relations in both. For the relations between the two sexes 

 cannot be adequately understood apart from parental functioning, 

 and often care. 



BIO-PSYCHOLOGICAL. — In discussing many physiological ques- 

 tions, and ecological ones yet more, as from the working of the 

 nervous system to the migration of birds, it becomes impossible to 

 ignore the psychical aspect ; and this is surely increasingly prominent 

 in mating and in mothering. Thus we are naturally led to a bio- 

 psychological inquiry, for the animal kingdom at least, — an arrange- 

 ment of the different levels of behaviour, from simple reactions to 

 reflexes, from obligatory movements to instincts, from the simple 

 "trial and error" of a food-testing spider to the surely intelligent 

 grasp of a situation by a sheep-dog or by a chimpanzee. Here must 

 be faced the perennial "mind and body" problem, and even that of 

 the not impossible role of mind as a factor in evolution. Such are 

 some of the diflicult tasks for the section called bio-psychological. 



MORPHOLOGICAL.— Side by side with the question, How does 

 this organism or organ work? there has developed the question, 

 What form and structure has this in itself, and in all its parts? This 

 seems a simple question, but how difficult to answer, as we press it 

 further and further home, from external features and symmetry to 

 internal structure, from organs to tissues, from these to cells and 

 the living matter that composes them. How intricate the answer 

 must become as the structural analysis deepens, as we put one lens 

 after another in front of our own, as we call to our aid all sorts of 

 devices — from scalpel and forceps, to begin with, to razor and 

 microtome, fixative and stain. Dissection, even of minute animals, 

 has been carried to a high perfection by a long succession of zoo- 



