More complex organisms cannot give rise to new individuals through regeneration primarily because their bodies are highly complex and organized into specialized tissues, organs, and organ systems. This complexity involves:

• High levels of tissue differentiation and specialization, where different tissues and organs perform specific functions and cannot simply regrow or form a whole new individual.
• The division of labor in their bodies with interconnected organ systems that function as a coordinated whole, making it impossible for any one part to regenerate an entire new organism.
• Regeneration relies on specialized cells capable of producing new tissues; such cells are present in simple organisms but not in sufficiently abundant or capable form in complex organisms.
• Complex organisms can regenerate some tissues (like skin or muscles) but cannot regenerate whole organs or organ systems, nor can they regenerate a new individual from body parts.

In contrast, simpler organisms have less differentiated cells, often similar across their bodies, allowing parts to regrow into complete new individuals. The specialized and organized nature of complex organisms prevents this form of reproduction through regeneration.