444 POPULAR SCIENCE MONTHLY 



not as the rungs of a ladder as Lamarck supposed, but rather as the 

 branches of a tree or a bush; and such branches again like those of a 

 tree bear smaller branches, and these reach to lesser or greater heights 

 from the base level of primitive organization. Thus, anatomy holds 

 that community of plan is an indication of genetic affinities, while 

 modifications of a common plan exhibit the results of adaptation to 

 different ends through evolution. The framework of the human arm 

 is constructed out of the same elements with the same arrangement that 

 we find in the leg of a cat, the flipper of a seal, the paddle of a whale, 

 and even the wing of a bat, different though these structures are in 

 function — and in these resemblances comparative anatomy discerns 

 evidence of a remote common ancestry of men and whales and bats. 



Extended through the study of tissues, or histology, to the unitary 

 elements of organic structure — the cells — comparative analysis has 

 brought the whole realm of organic nature under the sway of a great 

 principle — the cell-doctrine of the botanist Schleiden and the zoologist 

 Schwann. This important principle, propounded in 1838 and 1839, 

 produced an immediate effect in unifying organic creatures, though 

 many years passed before it was formulated in the terms employed 

 to-day. In brief, it is this : All the larger organisms are composed of 

 organs which in turn are constructed of various tissues, like muscle 

 and nerve and connective elements; the tissues finally can be resolved 

 into units of structure, the cells, which agree in possessing a central 

 body or nucleus, and in their protoplasmic substance. The elementary 

 nature of cells is still further demonstrated by the simplest organisms 

 we know, which consist of one cell, nothing more and nothing less; 

 while finally the starting point in the development of higher animals 

 is always a single cell — the egg. Truly these are remarkable facts, 

 when we consider the wide range of animal and plant forms. 



Vast as the present knowledge is, the tasks of comparative anatomy 

 are not entirely completed. Though voyages of exploration like those 

 of the Beagle with Darwin, the Rattlesnake with Huxley, and above 

 all of the famous Challenger have gone to all parts of the globe, though 

 countless investigators have devoted their lives to the study of special 

 groups like birds and mammals and insects and molluscs, every year 

 brings to light new forms that must be analyzed and placed; while 

 new discoveries in other departments often make it necessary to reex- 

 amine known series in the light of fuller knowledge. 



While many naturalists prior to the nineteenth century were inter- 

 ested in the way an animal egg produced an adult organism, it was not 

 until the doctrine of descent energized zoology that comparative embry- 

 ology attained the independent status that it holds to this day. Harvey 

 in 1650 had perceived that, in his own words, " all animals are in 

 some sort produced from eggs." Bonnet and Haller, of the early 



