322 ANNUAL REPORT S^^THSONIAN INSTITUTION, 194 



something like a book whose leaves can by incredible patience be un- 

 folded. Naturally the oldest records lie in the deepest strata, and 

 those are generally the most distorted and very difficult to read. The 

 record is not complete — there are distressing gaps. In some cases 

 Avhere one class of creature is known to have originated from an an- 

 cestral form, no intermediate or "missing link" can be found. The two 

 forms may coexist, and be traced back for a long period in this record 

 in the rocks, showing closer and closer relationship in structure, so as 

 to leave no doubt about the origin of the descendant form. Suddenly 

 in this working back, all signs of the later form cease, and earlier strata 

 will show no traces of it. Thus it sometimes seems as if the new class 

 of creature had suddenly appeared full-fledged and complete. In 

 other cases, however, the developmental or evolutionary record is for- 

 tunately complete. 



Beyond the disputes of scientists about minor points is the fact 

 that fishes of sorts were the ancestral forms from which all other verte- 

 brate creatures have originated. We have no record of any accepted 

 "link" between these fishes and their most likely invertebrate ancestors. 

 The fishes are suddenly there, some 370 million years ago. in numbers, 

 find in a diversity of weird forms. From the fishes originated the 

 amphibia, and from them the reptiles. These were all cold-blooded 

 creatures, very much at the mercy of sudden climatic changes. The 

 first amphibia appeared about 320 million years ago, and reptiles 

 evolved from them by 90 to 100 million years later. Those sluggish 

 creatures were produced by nature in great diversity of size and 

 shape, but most of the larger forms in specialized groups have 

 become extinct. 



The call for greater activity and mobility produced from the 

 reptiles the warm-blooded birds and mammals, the latter class, as 

 typified by man, being now dominant on the earth. 



To return to those early fishes, some of which were our ancestors : 

 Many of them, the only ones we Imow, left traces in the rocks. Usu- 

 ally the skeleton and any teeth, spines, or hard skins are preserved, 

 in rare cases perfectly. The "soft parts" are largely unknown, and 

 the reconstructed outlines of extinct forms are to some extent guess- 

 work, but no more guesswork than the diagnosis of appendicitis by 

 a physician. In each case visualization of the hidden condition is 

 based upon experience and Iniowledge. How close such visualization 

 may come to actuality may be seen on comparing the outline of a 

 Coelacanthid fish thus reconstructed (fig. 2) with the photograph 

 of the present-day fish. (pi. 1). 



The primitive ancestral form of fishes was almost certainly some- 

 thing rather sharklike, without any true bone in its make-up. From 

 those creatures in a relatively brief period of time evolved a multi- 

 plicity of types which are generally divided by scientists into four main 



