THE CAMBRIAN PERIOD. 289 



immediate indications, but philosophical geology goes further and 

 endeavors to supply such forms as may be necessary to fill out a con- 

 sistent assemblage of life. It has already been noted that the pres- 

 ence of so much animal life implies much vegetable life to supply the 

 necessary food. This inference is almost as firm as the inference that 

 the fossils really represent organisms that once lived, which was once 

 questioned, but is now regarded as a scientific conclusion. Firm infer- 

 ences, like that of the presence of abundant vegetation in the Cambrian, 

 grade away insensibly into others that are weaker and weaker until 

 they become very infirm. There is hence a danger of extending phil- 

 osophical geology until it shall pass into speculative geology, without 

 being fully aware of the fact. Great circumspection is needed when 

 the field of free inference is invaded, but the subject of ancient life 

 would be left barren of half its interest, and most of its stimulus, if 

 inferences were not indulged in their proper places and to their proper 

 degrees. The need of caution arises more largely perhaps from unrecog- 

 nized alternatives than from any other single source. For example, in 

 the inference relative to vegetation in the Cambrian and pre-Cambrian 

 times, which seems so imperative, it is barely possible that an early 

 form of life intermediate between plants and animals, or combining 

 their qualities, and ancestral to both, may have lived in the earliest 

 ages, and have possessed the power of organizing organic matter from 

 inorganic matter. These may have furnished the ulterior food of 

 the animals, instead of plants, which may have been differentiated 

 from them later. And so even the strong case of inferring vegetation 

 from the presence of animals is not without some small occasion for 

 reserve. But this is really only a change in the form of the infer- 

 ence rather than in its real substance, which is that Cambrian animals 

 imply the existence of plants or of unknown organisms that performed 

 the chemical functions now performed by plants. 



Duly mindful of the weaknesses and dangers of pressing inferences 

 too far and duly appreciative of their stimulative value when properly 

 controlled, we may inquire how far the forms and functions of the 

 known Cambrian animals suggest the existence of other animals whose 

 presence is not recorded. A large percentage of the known Cam- 

 brian animals were provided with shells, tests, plates, or other forms 

 of hard coverings. In the main these appear to have been protective 

 devices, and imply enemies or combative rivals against which the 



