Why did Louis Agassiz think environmental changes could not produce new species?

Louis Agassiz believed that environmental changes could not produce new species because he adhered to the doctrine of catastrophism, which posited that the organic world resulted from repeated, independent interventions by a supreme being.1

While Agassiz acknowledged the existence of environmental change, variability, and hereditary modification, he maintained that ordinary physical events—such as climatic shifts or geological changes—were only capable of causing extinctions, not the creation of new life forms. Influenced by Platonic thought, he viewed each species as a "thought of God" and argued that organisms arose through a series of special creations rather than through hereditary continuity or interactions between populations and their environments. Consequently, he rejected the emerging theory of evolution, viewing anatomical similarities not as evidence of common ancestry, but as "associations of ideas in the Divine Mind".1