This book is a continuation of my book, published in 1963 under the title Essays from the History of Comparative Anatomy to Darwin. Development of the morphological type problem in zoology." As the title of this new book shows, it covers about a hundred years - from the middle of the 19th to the middle of the 20th century. The literature for this period, one way or another concerning the problem of the morphological type, is large and diverse. Neither the time nor the amount of work allowed me to collect and discuss all the literature. I limited myself to trying to show the most important points and directions of development of the problem of interest to us, as far as I managed to find out them. In accordance with this, the list of references does not claim to be exhaustive. It contains mainly works mentioned in the text.