Preface 



This little book was written in 1942 but, as a consequence of 

 the war and its aftermath, the Dutch edition did not appear 

 until 1948. It was meant in the first place for readers who, 

 though interested in its subject and having some general know- 

 ledge of science, were not acquainted with more than the first 

 elements of biology. This determined the design of the whole 

 book. Highly technical digressions had to be avoided, and, as 

 far as possible, the construction of a well-rounded picture of 

 the phenomena of development had to be attempted. It will 

 be clear that this made it impossible sometimes to make a 

 sharp distinction between well-founded facts, and the more or 

 less hypothetical trains of thought which link them together. 

 However, all that was possible has been done to steer clear of 

 the major obstacle to all popularization of science: the danger 

 of telling half truths. As for relating the results of investiga- 

 tions, the author has tried to apply the same strict standards 

 as are used in a purely scientific publication. In one respect 

 only he has made a concession to the character of the work: 

 he is aware that the way in which the scientific terminology 

 has been used in this book will not satisfy rigorous norms. 

 This could not be remedied without confusing the reader by 

 introducing a still greater number of technical terms. 



In the preparation of the English edition, no essential change 

 has been made in this design or its realization. An attempt 

 has been made, however, to bring the book up to date by 

 taking into account the literature up to about the middle of 

 1952. This is not a textbook, but a list of references has been 

 added in the interest of those who might want to refer to the 

 original publications on some special problem. This list, too, 

 has been brought up to date in the English edition. At the end 

 of the book, a glossary of scientific terms has been included 

 for the benefit of those readers who are not professional 

 biologists. 



VII 



