Final answer:
Animals can be divided into two groups: endotherms and ectotherms. Endotherms generate internal heat to maintain a constant body temperature, while ectotherms rely on external thermal heat. Therefore, the statement is true.
Step-by-step explanation:
Animals can be divided into two groups: endotherms and ectotherms. Endothermic animals are able to maintain a constant body temperature in the face of environmental changes because they generate internal heat. On the other hand, ectothermic animals rely on external thermal heat to help regulate and maintain their body temperatures. Therefore, the statement that animals are either entirely endothermic or entirely ectothermic is true.